[matasar]

This is going to be wrong for some of your users, because some ZIP codes cross state boundaries. So you'd need to let them correct the guess in that case. It is unusual for one zip code to be in two states, but it does happen. [1]

It's also going to be wrong for military addresses, as I understand it. [2]

[1]: http://en.wikipedia.org/wiki/ZIP_code#By_geography

[2]: http://www.usps.com/supportingourtroops/addressingtips.htm

[zackattack]

Which zip codes span multiple states?

[showerst]

http://en.wikipedia.org/wiki/ZIP_code#By_geography check the unordered list at the bottom of that section

[matasar]

Getting a complete list is very hard due to the nature of zip codes, but the wikipedia article I linked to has an incomplete and unordered list.