After teaching high school students how to fill out fafsa information (and doing it myself for 7 years.. yes I said seven) let me try and answer some of these questions.
1. As others have mentioned, many schools require that you claim your parents. Normally, if you were applying for undergrad funds you would be an independent if you are over 24, married, have kids, or are the ward of the state (an orphan). For medical students there are two reasons that these rules go out the window. The first is, most medical students have zero money themselves and will not be working during school. Second, every medical student will get some kind of financial aid. Most will be in the form of loans, but everybody gets the help they need. By having you claim your parents, it helps schools determine who really needs the relatively few scholarships and school grants available. Since on paper, every student would look poor and needy, there is no other fair way, but to look at the parents income.
Take home message: The FAFSA will say that you are an independent, but will also ask if you want to include your parents information. Put yes, unless you are 100% certain you will be attending a school that does not require it. It isn't really going to hurt you unless your parents are EXTREMELY well off. You will get aid for medical school. You may just have to pay more interest in the long run.
2. Also, you HAVE to use tax information from 2006 (the taxes that are due this April). If your parents haven't filed... it doesn't matter! Still submit your FAFSA. There is a question that asks if they have filed, just say that they WILL FILE. You just need guestimates on everything. If they want proof later, believe me they will ask. If the numbers are close, but not exact, they will not hold it against you. They will just adjust the aid accordingly.
3. List all school codes that you might be attending. You can change them later. I think there is a max of five that can be listed. You can change the codes to the schools at any time. So for now put what schools you were accepted to and if you have room (or don't have an acceptance) any schools you interviewed at.
4. Also, put off-campus for each school if you are not sure where you will be living. They give more money for off-campus. If you end up living in student housing, they will adjust the amount given to you. But, for now, reserve the max amount of money that you can.
If you made it through all of that.... Good luck!!