Here's my advice, because I was in the same situation you are in. Call your local wal-mart, ask to speak to the returns manager, and say you have a PS3 with the box and everything (I hope you do,) and that you got it as a gift, but wanted something else instead. They will take 3 items for return through the course of a year without a receipt.
They will have to send back the PS3 to get refurbished probably anyways, but they will give you the full amount that it was last sold for. In my case that was $523 for my 80GB Motorstorm PS3. They do give it to you on a gift-card, but come on, who doesn't shop at Wal-Mart?