What are the negatives of jailbreaking an Iphone 3GS on ATT?

I’m pretty sure jail-breaking voids your warranty, and you run the risk of bricking your phone at some future date, not to mention exposing your phone to potential malware attacks. There was an iPhone virus that was released in Australia I believe that affected people with jail-broken iPhones who never bothered to change the default password.

There are many compelling reasons to do it, just be aware that there are potential hazards which could be costly.