It almost goes without saying, but we will say it anyway :)
With all of the great stuff lined up for us with the 3.0 OS that Apple described today, many 3G owners may find themselves with itchy update fingers. If you find yourself with access to the 3G IPSW for 3.0 via the iPhone Dev Center program, and you are using yellowsn0w, do not update or restore to that official IPSW. You will lose yellowsn0w and find yourself unable to revert the baseband to get it back.
And for those wondering, yes the 3.0 OS is jailbreakable on all devices. It’s just those using 3G yellowsn0w that have to show some restraint and wait for PwnageTool to create a custom IPSW that avoids the baseband update.