Hi -

I recently purchased an iPhone 3GS with the old bootrom, but the phone has never been jailbroken or unlocked. I want to do just that, so that I can use it on T-Mobile.
Is there a way I can unlock and jailbreak this phone? I tried doing a restore to 3.1.3 with the ipsw found here (iPod, iPhone and iPad Firmware Download), but iTunes gives me the following message when I try to do it: "the iphone iphone could not be restored. this device isn't eligible for the requested build". I also changed my hosts file to point it to the cydia server, but that didn't make a difference.

Any help would be greatly appreciated.

Thanks!