Why can't my iPhone connect to any wifi networks?

I have an iphone 3GS. I don't use it with a phone plan; I was really just using it for the wifi apps. One day the wifi stopped working. It recognizes the wifi network but won't login anymore, not even to open networks including my own. I have reset everything, and erased everything including my own. I have reset everything, and erased everything several times. I bought a new sync cable and tried syncing it again. Still no luck.

My phone is not jailbroken and I have been hesitant to do that. Some forums have talked about it possibly being a problem with one of the updates in the software itself but I really don't understand how to go back to an older version of the software. My phone is not underwarranty.