Anticipating Apple releasing the iPhone 3.1 update at or following tomorrow's "It's only rock and roll, but we like it" special music event, the Dev-Team has posted a reminder:

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).

Rather, their PwnageTool should allow you to update the firmware without updating the baseband, preserving ultrasn0w for the "best of both worlds".

So, jailbreakers and unlockers, no rushing to out to download that shiny new 3.1 software via iTunes. You've been warned.