I guess lucked out, cuz I got in on this deal when I bought my iMac on Black Friday two months ago. Got a $99 printer for free.
Didn't have to mail anything in, and in fact, I don't remember filling out any forms online either. A couple weeks after my purchase, I received a check from Apple for $100. Easy peasy.
Completely painless, compared to all the other rebates I've ever gotten before.