With the release of iPhone OS v3.1 expected tomorrow in coordination with the Apple "It's only rock and roll, but we like it" event, the Dev Team has just updated their blog with a warning to all using a jail broken iPhone.

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).

So for now, just SAY NO to the initial iPhone OS upgrade and we'll wait for more word from the Dev Team!