I am on AT&T and have an iphone 3G. Are there any benefits to me unlocking the phone? Will I jump on a t-mobile signal if it is available and an at&t signal isn't?

No, you won't get "T-Mobile" on the carrier name with an AT&T SIM. It won't just hop over.

There are many benefits to unlocking the phone, but if you don't see it necessary, it's not needed. If you make that random trip to Canada/Mexico, need to lend your phone to someone on T-Mobile, etc. it can come in handy.

I travel to China a lot, so I require an unlocked iPhone. That's just one of the examples.

__________________
iPhone 2G T-Mobile USA
17 November 2007
1.1.1-->3.0 (jailbreakme.com, anySIM, iJailbreak, ZiPhone, PwnageTool, QuickPwn)

iPhone 3G AT&T/China Mobile
5 January 2008
2.2-->3.1 (yellow/red/ultrasn0w, PwnageTool, QuickPwn)

I can try to answer your iPhone related questions! follow me at @acwan on twitter!