Taken from blog.iphone-dev.org
You’ve seen us give this warning before, and there are only so many ways to say it or come up with a clever title 
But here it is: ultrasn0w users must stay away from any firmware updates past 3.0 (including today’s 3.1 beta) until we release the tools that let you update the firmware without updating the baseband. For most phones out there, baseband updates are irreversible and you’ll lose ultrasn0w.
The 3.0 jailbreak was one of those (rare) times where both the jailbreak and the unlock coincided (the only other time was 2.2). It’s important that people realize that *most* firmware releases aren’t like that, and you need to take steps (via the tools) to separate the firmware update from its included baseband update.
This warning does not apply to the iPhone 2G, which uses BootNeuter for the unlock, not ultrasn0w.