Just read the instructions at apple.com/promo. Hit Submit a Rebate and go for it. It took two weeks for me to get my Nano rebate and I just got my printer rebate two days ago. I sent them on the 2nd of July. Pretty good.
Even if you do it online you still have to mail everything in. Except you save time on processing that's all.
Definitely make photocopies of everything for yourself, cut the UPC's out of all the layers of cardboard, include originals on everything and send it out certified signature required (if available in Canada).