I currently have a jailbroken and unlocked (t-mobile) ver 1.1.1 (3A109a) baseband 04.04.05_g and I think I'm maybe on 1.1.3 or 4, I'm not entirely sure... it was done so long ago.

I've been putting off upgrading for a long time now, was afraid to do the 2.0 jump, but now that the Dev team has a sim unlock I'm tempted again.

Is it possible to upgrade my baseband and get to the point to where I could unlock and use yellowsn0w with success?

No. Yellowsn0w is for iPhone 3G only running baseband 2.28.00. Use pwnage tool or quickpwn instead

Oops... Ice Age...

2.2 has been unlocked since ...... I don't remember, a lot ago.

You can use quickpwn of you're in Windows and PwnageTool if you are in mac to unlock 2.2, it works just fine, and yellowsn0w is just for 3G!

If you need further assistance you can send me a PM.