This question could be rephrased as : Of the people picked so far, what is the ratio of people from 'good schools' vs people from 'bad schools'? Is this information publicly available?
But, it's easy to say that a lot of the YC company founders that I know came from the following schools:
Stanford (by far the leader, I'm sure)
MIT
Oxford (a surprise to me)
CMU
But, a lot also came from other places. I can think of examples from UPenn, University of Florida, many of the UC schools, Monash University (in Australia), as well as folks who don't actually have a university degree (me, for example--I went to a bunch of different schools for a bunch of different subjects, and none worth speaking of).
I suspect the "disproportionate" numbers are due to the basic fact that folks who go to an "elite" university are more likely to be highly ambitious, and school was just another way to achieve. Motivation plays a big role in getting into YC...and getting into and graduating from a top university takes a lot of ambition and motivation. I don't think YC thinks a lot about the college field on the application. If they did, Virtualmin wouldn't be a YC funded company; though my co-founder did go to a good school, I didn't know what it was when I filled out the application, so as far as YC could tell we were both marginally educated.
Off the top of my head I know of 5 founders in the current group of 50-60 who went to Ivy League colleges. There may be more, but I'd have to go back and look at the applications to know how many.
But, I doubt it is considered a vital statistic...just another way to indicate motivation and ambition. I'm pretty sure pg and Co. know that university is not where most of the best hackers acquired their skills.