If you have not known the latest news on iOS world, it's reported that the White hats hacker, Charlie Miller found security vulnerability in iOS 5 which can be exploited by black hat hackers for malicious purpose.
After the finding, Apple banned Charlie Miller's app developer license, and suspended him for one year from Apple dev program. It's because Apple regards him has violated the developer agreement.
Therefore in the upcoming iOS 5.0.1, Apple will be presenting a firmware that already been guarded to minimize all the security holes for any potential hacks, including for untether jailbreak.
So stay away from iOS 5.0.1 update, if you are planning to jailbreak iOS 5 for your iOS device.