You know, having just filled out the rebate for the free printer (which is the same process-same fulfillment house), I can say that it really is pretty lame Apple doesn't have a more convienent system for this. Any informed consumer knows that the whole concept of mail-in rebates is somewhere between shady and an outright scam depending on how it's handled, and as much as I'd like to think Apple is "better" than that, they're not.
What burns me most is that they KNOW the exact computer you bought from them, including the serial number--it's on the invoice. Yet you must cut out the entire product code label from the box--those beautiful boxes Apple spends so much time and effort developing--and physically mail it to the rebate house, even after you've filled out the online portion of the form. I could care less about cutting up my printer box, but the MacBook Pro box is another matter. Not to mention that they tell you to send in copies of your recipts, but don't make it at all clear what this means if you order online and the only recipt you get is via e-mail.
If they really wanted to give an honest discount, they'd just make it an "instant" iPod discount when you bought the two together at an Apple Store or the online store. Or, if that's just too much "free" for the bottom line to handle (the whole point of the mail-in hassle, after all), they could AT LEAST let you just register the whole thing online using the serial number of the computer they know you bought, and maybe force you to mail in a signed form just to increase the hassle (and, ostesibly, to have a signature to "prove" that you really did buy the stuff).
It's ironic trying to "prevent fraud" by making you hack up your box when they know better than you do the serial numbers of the computers (and I expect even printers) you bought from them.