Advisory from Dev-Team:
"This week Apple will be all over the news with their announcements at Wednesday‚Äôs ‚ÄúLet‚Äôs Rock‚Äù event. But with so many new owners of the iPhone 3GS, and with so many new owners of the iPhone 3G (perhaps sold to them by these new 3GS owners)‚Ä¶now is a good time to send out this general advisory.
If you update to Apple‚Äôs new software using the normal iTunes process, you will lose your ultrasn0w unlock. In fact you may lose it permanently, because for most people the baseband firmware cannot be reverted to a previous version (unlike the main application CPU firmware).
But don‚Äôt worry‚Ä¶our PwnageTool program lets you update your main firmware without touching your baseband firmware, so you can still have the best of both worlds. But you must be diligent about saying ‚Äúno‚Äù to your iTunes request this week to update your firmware."
Update: if you are 3GS owner you will not be able to roll back to 3.0 or 3.0.1 after upgrading to 3.1. Read more: Jailbreak Heartbreak: Apple 86's OS 3.0 Restore