I know that the 4.2 iOS is out now, but I was wondering if I could get my jailbroken and unlocked iPhone 3G updated to 4.1 and then re-jailbreak and unlock it so I can use it on T-Mobile (Like I'm doing now) but now with updated firmware.
Yes, like said above if you update to official 4.1 you will lose your unlock and your Tmobile sim wont work.
You can update to a custom JB 4.1 IPSW created with pwnage tool so you can maintain your baseband and stay unlocked.