iOS 4.0.1 blocks the exploit used by ultrasn0w to decode phones on the basebands 04.26.x – 05.13.x so if you update to iOS 4.0.1 and update the baseband then YOU WILL NO LONGER BE ABLE TO DECODE THE PHONE! The warning came from iH8sn0w, but I'm sure that the Dev Team will soon announce the same thing, so be careful not to update to iOS 4.0.1 because you'll be left with a brick on the table.
Well, the joy of iOS 4.0 didn't last long...
Along with iOS 4.0.1, iOS 3.2.1 for the iPad appeared, which blocks the exploit used by Spirit to jailbreak the tablet, so if you update to iOS 3.2.1, you will no longer be able to jailbreak the iPad.
UPDATE : Looks like iH8sn0w was mistaken, MuscleNerd a confirmed that ultrasn0w works on iOS 4.0.1.
