I believe you have 14 days to return. It may affect your contract with ATT though, unless you already had ATT. You can return it to Apple, cancel your contract, get a full refund, and buy the 16gb-check with others though.
i had the iPhone v1, when i bought the 8GB i thought i could get the white in 8GB so i only brought enough to buy that...then i find out white is 16GB only, so instead, i bought the 8GB but now want the 16GB white..ill call Apple and see. thanks