This 2003 article from Christian Century provides some good information. Here's a USA Today piece that Fr. Fox cited.
In sum, Protestants do not have a clergy shortage. However, they do have a lot of churches without ministers.
In the denominations often designated as liberal the Episcopal Church, the United Church of Christ and the Presbyterian Church (U.S.A.)--roughly 20 percent of churches lack clergy (according to Yearbook data). The "moderate" mainline denominations--including the United Methodist Church, various Lutheran denominations, the Disciples of Christ, the American Baptist Churches and the Reformed Church--show a 10 percent vacancy rate. The conservative denominations, which have among the highest numbers of clergy per member, also have the highest proportions of employed clergy per church. The data from these denominations--including the Southern Baptist Convention, the Church of the Nazarene and the Assemblies of God--suggest that there are 1.4 working clergy per church. (Anecdotally, however, officials in these denominations believe that there are still empty pulpits out there, and they estimate the percentage to be between 4 and 6 percent.) [source]And yet the ratio of ordained Protestant clergy to congregations in the country is 2:1. Liberal denominations have both a high vacancy rate (significantly higher, actually, than for Catholic parishes) and a lot of unemployed clergy.
The problem seems to be that while there are a lot of ordained clergy, many of the openings are only marginably able to support a minister. The PCUSA reports that 50% of their vacant churches have congregations of less than 100 adults, many of whom offer salaries in the 20k range for ministers. Many of these are also in rural areas, making it difficult for a minister supported by his or her spouse's job to fill the vacancy. Denominational polls of ordained Protestant clergy revealed very few were willing to go to rural areas, or in some cases to move at all, since in many cases they relied on a second job or a spouse's career to support their families.
One of the major contributing factors here is obviously the Protestant model for ordination in many denominations, where potential ministers will attend a seminary and be ordained, yet in essence be independent contractors hired by individual congregations, rather than reporting directly to the authority of a central hierarchy and being assigned wherever needed.
While clearly, if we had two priests for every parish in the US we would not have a clergy shortage, the nature of the Protestant 'shortage' shows us some of the problems we would invariably have if a married priesthood became widespread in the US. Transferring priests, especially to undesirable areas, would become much more difficult -- whether because the priest's wife had an established job where he was currently stationed or simply because he didn't want to take his family into an undesirable area. Married priests would have to be given much more independence than our current priests have, and celibate priests used to fill in the 'undesirable' slots, relegating celibate priests to what would look and feel a lot like a second class citizen status.
It would also be nearly guaranteed that we would have a higher percentage of 'drop-outs' among married priests. Between long hours, low pay and emotional stress, the priesthood would put a lot of strain on a priest's family, and (as with any other very stressful and consuming job) married priests would sometimes come to the conclusion that they either needed to change careers in order to save their marriages or take a few years off or part time. This works all right in most Protestant denominations, where the concept of the sacramental priesthood is not as strong or non-existent. But for Catholics, having a priest 'go inactive' or 'quit' is a much bigger deal. In the Catholic view, the priesthood is something you are, not something you do.
As I think about it, though, I can't help wondering if the argument over a married priesthood is actually something of a proxy fight over what sort of priests people envision as 'the right sort'. Catholics on the more conservative end of the spectrum tend to insist that a more conservative approach to theology, liturgy and vocations will increase vocations and point to places like the Lincoln diocese for evidence. More progressive Catholics tends to reply that in cases like the Lincoln diocese "the odds are good, but the goods are odd".
Now, the thing that few people seem to have brought up yet is that, to the extent that a widespread use of the married priesthood would be seen as a major shift in the progressive direction, it would probably dis-incent a lot of more conservative men from joining the priesthood. When you open a group previously open exclusively to one pool of members to other possible members, you change the joining patterns of the original source pool.
An anecdotal example: When I went to our parish grade school back in the late eighties and early nineties, about half the boys in the 4th through 8th grades were alter servers. It was still open only to boys, and the joining rate was pretty high. Not long after I went to high school, the instruction came down that girls could now be alter servers as well. The result was that the number of alter servers fell from around 100 to about 40. Why? The boys weren't as interested in something was was no longer exclusive to them, and not many girls were actually interested in being servers. Opening to the wider pool changed the behavior of the original pool of applications, so even though twice as many people were eligible to be alter servers, only half as many people actually joined.
Now, one would assume that people considering the priesthood are a good deal more mature in their reasoning than we alter boys were. And yet, there is the very strong possibility that opening the priesthood to married men would have the effect not of increasing the number of vocations, but of having the same or even a smaller number of vocations, but from a different group of people.
Obviously, one can have a married priesthood: the Eastern rites have done so for centuries. But today's Church is one already badly bruised by good changes implemented in a hamfisted fashion. My biggest fear in regards to the idea of opening the priesthood entirely to married men (as opposed to the small number of dispensations we currently have for converts) is that rather than increasing the number of vocations, it might deal the priesthood an even more grievous blow than it has experienced to date.
UPDATE: As I think about it, it seems like the non-destructive way to test the married priest idea in the West would be to allow an order or other non-diocesan confraternity to form made up only of married priests. This would avoid the two-layer priesthood that would doubtless form at the diocesan level with married priests, and the order or confraternity could be assigned to staff certain parishes in order to help keep parishes staffed. Bishops who didn't think it would be healthy to have married priests in their diocese could choose not to grant the order any parishes. And the nationwide or worldwide structure of the order would allow it to assure that it's priests were paid enough to support their families, or placed in cities where their wifes had work.
This would also allow the faithful to get used to the idea in a limitted setting, and allow the heirarchy to see if there was in fact a significant pool of married men interested in the priesthood -- and how they did as priests.
I'm not necessarily agitating for such an order, because, frankly, I don't think ordaining married priests in the Roman Rite would work all that well. But if it was going to be tried, I think that's how it should be done.