What is the policy regarding college admission for the mentally retarded?

Colleges are certainly interested in diversity. Admissions committees are likely to select a class of people of mixed background – varied geographic locations, varied ethic backgrounds, etc. I believe they may also show favor toward the physically handicapped. But what about the mentally retarded? Does admitting a mentally retarded person add to diversity in the same way? Is it considered discrimination to reject a mentally retarded person because he/she does not have the proper test scores / demonstration of ability? Are mentally retarded people judged on different criteria in order for them to have a better chance at college?

[I ask because of an experience I had yesterday. I am a grad student at a very prestigious university, and I use human subjects in my research, which all come from the undergraduate student subject pool. I had a subject yesterday (who is a student here), and I am almost positive he was mentally retarded. The class of incoming freshman here this year had an average SAT score of 1470, so this isn’t adding up for me.]