Here’s a warning from the Dev Team about iPhone firmware 3.1.
“This week Apple will be all over the news with their announcements at Wednesday’s “Let’s Rock” 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 buy these new 3GS owners)…now is a good time to send out this general advisory.
If you update to Apple’s 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’t worry…our PwnageTool program let’s 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 “no” to your iTunes request this week to update your firmware.”
Source: Dev Team