How can I use my iPhone 3Gs on T-Mobile? Currently my iPhone 3Gs is on iOS 5.01.  I'd like to use it on the T-Mobile network.  What are the steps required to make it happen?

I think it's obvious that I need to jailbreak the phone.  However, googling around around it is unclear whether there is a reliable (untethered) jailbreak for 5.0x.

Do I need to downgrade the OS?  If so, to what version and how?
Any additional steps I need to take?
Tethered jailbreak is available for IOS 5, see Jailbreak iOS 5 5.0/5.0.1 on iPhone 4 iPod Touch iPad with Redsn0w and of course blog.iphone-dev.org.