Well that was quick as ever. The always-punctual iPhone Dev Team's already jailbroken (tethered) iOS 5, which has only been live a few hours, for the iPhone 4, iPhone 3GS, iPad, and iPod Touch. iOS 5 has been jailbroken since around the time of the initial beta, but this is confirmation for fresh installers.
As always, you should know that the tethered version, which means you have to connect to a computer running Redsn0w every time you turn on your device, will probably require you to restore to a previous backup if you want to install an untethered jailbreak once it's released.
If you rely on Ultrasn0w or Gevey SIM for a carrier unlock, you shouldn't upgrade to the stock iOS5, and instead follow the instructions in Redmond Pie's guide to work up a custom firmware file, although the iPhoen Dev Team still recommends you stay away for now. The jailbreak also won't work for the iPad 2 or the iPhone 4S, if you've magically got one of those already.