Yes, the iOS 5 beta which Apple announced has been jailbroken within 24 hours of its release. It was DevTeam who JailBroken the first developer beta version of the popular operating system on iPod touch 4G. A screenshot is found on Musclenerd’s twitter account that shows root access through iSSH which indicates that the device has already been JailBroken.
Right now the exploit should work with most recent Apple iOS devices, but not the iPad 2. MuscleNerd has also only confirmed that a tethered jailbreak is possible. That means you’ll need to connect your phone to a computer via a USB cable and run the software again any time you need to reboot your device.
It’s probably just a matter of time before the developers behind redsn0w, sn0wbreeze, or another popular jailbreak utility come up with an untethered jailbreak method. Just don’t expect it to hit the streets before this fall when Apple officially releases iOS 5 to the public. Jailbreak developers tend to hold off on releasing new software during iOS beta test phases, since it would give Apple more time to make changes before releasing the final software.