Hi some people say it can work without problem on att or tmob, some people say that when unocked it can damage some software in the phone that can damage the phone....ebay says it can work wo problem.
One more thing to consider and this may not be a deal breaker for you, BUT the Storm would not be 3g on ATT or Tmobile, best you would get is Edge speed. My old work BB (8700) worked just fine on ATT's Edge network. If you want 3G, then the Bold is probably the way to go.