That's not entirely correct. A doctor's note does nothing, and the Disney folks will generally ignore them -- Disney follows the ADA guidelines of dealing with a person's ability to function, not a diagnosis.
What they will do, based on functional limitations, is issue a "guest assistance card" (GAC) with certain stamps/endorsements that allow the cast members at each attraction to know what accommodations might need to be made for the guest with a disability. I believe this is done on-site, and cannot be done in advance, because they issue the cards based on observable conditions.
It's not a "go to the front of the line" pass, but it could be used to, say, allow a stroller to be used as a wheelchair, and so be allowed in line, as an example.
In our family's case, despite the twins both being diagnosed as severely autistic, their ability to function as far as going to a theme park is concerned, is very minimal. Our daughter has no issues that could be addressed by a GAC, and our son is likewise mostly functional, but would need seating in shows near an easily accessible exit so we can get him out in a hurry if he has an "episode". Other than that, they're fine with the park environment and the rides. We avoid long lines anyway via fast pass and getting there early in the day.
http://disneyworld.orlandovacation.com/articles/special-needs/guests-with-autism.php
Deb’s Unofficial WDW newsletter, AllEars, is an excellent resource for anyone visiting Disney, contains tips, discounts show information, accomodation reviews advice on closures and remodeling and reader comments on all experiences with all aspects of Disney. A clue is to know the resort poilicies before you go, prefereably call first.
http://allears.net/pl/dis_aut2.htm
http://allears.net/pl/dis_aut3.htm
http://allears.net/pl/dis_aut4.htm