An article in the November 4, 2010 Education section of The New York Times, details the latest information on applying for accommodations on both the SAT and ACT. The most common accommodation requested is extra time. Students with learning disabilities or ADHD can receive such accommodations on these exams, but the process is complicated and families need to prepare well in advance.
Both the SAT and ACT require a "paper trail" detailing the need for such accommodations including evaluation results from a qualified psychologist, as well as evidence of similar accommodations being provided at the student's school.
Please note that both testing organizations are skeptical of students who do not receive either testing or accommodations in their in school prior to high school, since they assume that in such cases, students are trying to "game the system." Therefore, if students and their families are considering applying for accommodations on these exams, it really pays to plan ahead and not wait until high school. For more information, read the complete article here.