If you update to Apples 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.