I know colloquially that the applicant pool was MUCH bigger this year than last year. We might be able to open up a couple more spots, but definitely not in proportion to the increase.
Not so sure about this. While programs have shared admissions information for years, particiapation was voluntary and the number of programs that did this varied. This changed 3 years ago when AAMC got involved in the effort. The data is still not perfect, but we are begining to get a sense of the size of the applicant pool. There are about 1500 individuals that apply to at least one MD-PhD program each year. I believe that individuals are applying to more programs than before (probably due to the fact that many MD-PhD programs now do not require anything more than the MD-PhD essays on the AMCAS, thus making it easier to apply to more programs); individual programs reporting more applicants does not mean that there are more applicants in the overall pool. Finally, the growth in the number of federally-funded programs does not mean that there are more MSTP slots. In fact, the number of slots has decreased due to the real-dollar decline in the NIH training budget. Nonetheless, I do believe that programs are holding steady in terms of the number of MD-PhD students they enroll each year by scraping together money from other sources to replace the slots cut by the NIH. If, however, the NIH training budget does not grow soon, programs may be forced to reduce the size of their entering classes. (We are all looking forward with hope to January 2009, when Crawford, Texas welcomes home its village idiot.)
Without definite data to back this up, my intuitive sense is that the number of MD-PhD students is greater than it was 10 years ago, but has been more-or-less constant for the past few years.