You must have your 3G iPhone activated with AT&T in the US. I'm surprised that AT&T hasn't sent you a notification of some sort...
The way I read it, it seems that the OP used to be subscribed to AT&T, and presumably used the iPhone on AT&T's network for some time. But for some reason the OP has since been forced to drop AT&T's service.
Presuming the OP either paid the ETF, or else AT&T chose to amicably waive the fee, the iPhone would then turn into a totally legitimate paperweight, but the OP would not (yet) have violated any agreements. As of that instant, AT&T would no longer have any standing in regulating the OP's affairs, so they wouldn't have any business sending the OP notifications of anything at all.
While the iPhone is in such a state, it is totally normal for it to be nonfunctional. It would be difficult to justify any warranty service under those circumstances, because no repair could be performed which would change the fact that without an active wireless service contract, the phone is nonfunctional by design.
Re-activating it on AT&T, or giving it away to somebody else who activates it for himself on AT&T, would almost certainly bring it back under warranty coverage for whatever term remains.
On the other hand, unlocking the iPhone and activating it on T-Mobile's network ... that's something which would permanently void your warranty due to this clause in the iPhone warranty:
This warranty does not apply: (...) to a product or part that has been modified to alter functionality or capability without the written permission of Apple.
(Original iPhone warranty page 1, Exclusions and Limitations section, paragraph 3 part e
link)
(iPhone 3G warranty page 2, paragraph 1 part g
link)