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.
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 iPhone 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.