The most important thing to note is that you still pay the list price for the camera, and you then have to reclaim the cashback amount from the manufacturer. This is supplied usually in the form of a cheque within a period of around 30 days. There are quite strict conditions. The camera must have been bought new from a recognised dealer, be accompanied by suitable proof of purchase, and remain in your possession (so that people don't immediately sell them on after claiming the cashback).
The other thing to look out for is that dealers will frequently quote the price of the camera after the cashback has been taken into account. They are obliged to point out that the amount includes the cashback, but this may be in the small print.
Cashback deals do offer the savings quoted, but you still have to pay the list price, you have to go through a few hoops to get your money back, and you don't get it straight away.
Nikon's Xmas 2010 2010 cashback offer in the D3100 is a good example of how these schemes work, and there's a cashback FAQ on this page too.
