1) You should rank every single program you interviewed at, unless you would rather be unmatched than go to a specific program.
2) You should rank the programs in the order that you hope to match to each one. The NRMP algorithm is skewed in the applicant's favor; it will not hurt you to rank your favorite program first, even if you think you have no chance of matching there.
3) The rank list you submit and certify on the NRMP website is a binding commitment. If you match to a program on your list, that's where you're going.
4) The deadline for submitting and certifying your rank list to the NRMP is 9:00 PM Eastern time on February 22, 2012. However, I strongly suggest that you certify your list well in advance of that deadline. Power failures and bad Internet connections have caused trouble for applicants in the past, and if you don't meet that deadline, there is nothing that anyone can do for you.
UPDATE: Yesterday, I posted the text you see above, and today the NRMP website went down for over two hours (see screenshot below), but they did not change the final rank list deadline. Imagine if you had only set aside those two hours to work on your list!