I don't know about "next to impossible" but I have heard that it is quite a bit more difficult to obtain accomodations for the USMLE than for the MCAT. The reasoning, presumably, is that disability or no disability, you are expected to perform "up to par," as a soon to be physician. (My interpretation of their stance.) I am sure, though, that in some cases, they would be willing to give accomodations. But they probably require a lot of documentation w/ evidence of formal psychologic/psychiatric testing.
There were some posts on this topic not that long ago in this forum, which you can probably find if you do a search under ADHD. Some guy mentioned that for ADHD it wasn't too tough to get accomodations for the MCAT but that the USMLE folks make it a lot toughter and usually refuse unless you have documentation showing evidence of it before age 7. (This is in line with DSM-IV requirements for diagnosis of ADHD, so this specific example isn't as draconian as it might seem.)
