Display models at stores like Best Buy, Futureshop, Target are often left sitting with no lens on and no body cap. As a result they can be full of dust and have fingerprints/scratches on the mirror or focus screen. Check it over carefully, and if you see problems contact Canon and see if you can send it in to get cleaned/repaired under warranty. If Canon won't touch it then return it.
To see how dirty the sensor is take a shot of a blue sky at f/16 and look for black blobs. With the lens off look for fingerprints on the mirror. If it's a mess don't have anyone at Target try to clean it.
I wouldn't bother with the 3 year warranty since if you're really concerned you'd be better off returning it and just buying a new in box one from B&H for $549.