Correct. I just got a new iBook for a student (Education Discount which was $100 off), but any item including printer, case, even the iPod offer with rebate means you have to order these items along with the initial purchase so that it appears all on the same purchase order.
I know for a fact that the printer has to be on the same receipt. I'm not sure whether or not you get the full $100, although retail corporations tend to be rather thrifty, and I would assume that you get a free printer and no extra cash.
On our previous printer rebate, the printer cost $99.95 and our rebate check was for $100.00. So while it's technically a case of the printer being cheaper than $100 and we still got a $100 check (woohoo, 5 "free" cents!), I'm not sure how informative this would be in the situation of a printer costing even less.