A note to add on to what @shilolo says. As the husband of a photo technician who handles such prints, make sure that the pictures are a decent resolution!
This probably isn’t an issue with an iPhone, but I know for a fact that the only thing that pisses a photo tech off more than getting a 640×480 cellphone pic (0.3 megapixels) and asking for prints larger than wallet-size is when the customer complains that the pics look like they were done with Legos due to the huge pixels. Real life is not CSI; photos do not have infinite resolution!
Now, the iPhone has a decent enough camera (2 MP) that you could might get some good-looking 4×6” prints, but if you go to enlarge them then they will look like crap. You might get away with 5×7”, but even an 8×10” will get iffy.
My point is this; don’t expect greatness from phone pics unless you have a phone with a better camera than the iPhone has. The HTC Droid Incredible has an 8-megapixel camera that can yield a great 16×20” print, and the Motorola Droid has a 5MP camera for a good 8×10” print. The iPhone…. well, just because it takes pictures, that doesn’t mean that it’s a camera.
See here for ideas on how to translate megapixels to print size.