Unlock possible?

[HONDAxACURA]

Hello! I want to unlock my old iPhone 3G for use with T-Mobile. I currently have... I already installed a jailbroken 4.2.1 on it already and installed cydia.

iOS 4.2.1
Modem Baseband 5.15.04
Bootloader 5.09

I've been researching on the possible ways to unlock this iPhone. I really want to downgrade it to 3.1.3 then unlock. But I'm guessing I can't do this because of the baseband. I know about upgrading to 6.15.00 baseband, but I don't want to risk it all, unless there's a way to go back.

What are my possible solutions right now?

I'm new to this iPhone hacking. I'm more familiar with the Android. Help please?!

 

[Sine Qua Non]

It's not unlockable through a software hack, and you can't downgrade it.

Your options are:
1. iPad baseband. You've already ruled this out. Good call, IMO. No way to reverse this unless a higher BB comes out eventually that you can upgrade to.
2. Gevey SIM. These are cheap now. Read into this more.
3. Factory unlock provided by one of those sites that have popped up in the last few days. Expensive. Also read into this more.