So as of today, Apple has stopped signing 3.0 and 3.0.1 restore files. That means that if you have a 3GS iPhone you cannot restore any firmware other than 3.1. If you are happily jailbroken today and something goes wrong you are pretty much screwed. Here is a summary:
1) If you have an iPhone 3g or 2g, you are fine and should be able to restore.
2) If you have a 3gs, you can only restore 3.1 (and kiss your jailbreak good bye). If you did not get an IBSS/IBEC signature before now and you restore, you may never be able to jailbreak again.
3) If you have an iPhone 3GS and you used Cydia to save your TSS or did purplera1n or similar a while back and saved your IBEC, then you will be able to restore in the near future, but you must wait for someone smart to write a utility that will let you do it. Chances are good that such a utility will come out in a couple days. If you can be patient and wait, do so. No one wrote this utility prior to this because we were not sure if Apple would actually stop signing 3.0 – only that they *could*.
It looks like an exploit has already been found in 3.1 firmware so it sounds like a jailbreak will be possible, but again, hold out for a bit.