How can I deactivate an jailbroken iOS? Sometimes the activation option from the PwnageTool does bring your GSM in an unusable state if you have and iPhone that is locked to other network than AT&amp;T.

All I remember is that it was a tool that I installed using Cydia that was able to deactivate the phone.
You should disable activation (it's hacktivation) when jailbreaking if your phone is unlocked (or locked to your carrier).

In PwnageTool you should choose Expert and deselect the Activate the phone checkbox.