Alright so it yesterday the dev-team released a new version of Ultrasn0w (1.2.1 which allows you to unlock your iPhone 3GS or 4 on 4.3.1. But don't get your hopes up too high. This unlock is just for compatibility with 4.3.1. So if you haven't preserved your baseband, then your screwed. As of right now Ultrasn0w is only compatible with these basebands:
3GS: 4.26.08 5.11.07 5.12.01 5.13.04 6.15.00
You can check your baseband by going to Settings --> General --> About. The baseband number will be under "Modem FIrmware". If your baseband is one of the ones listed at the top then your set!! you can unlock with the new Ultrasn0w! But if not, sorry but your SOL.
Heres what the dev-team wrote on their blog:
"We’ve pushed out the 4.3.1 compatibility fix for ultrasn0w in Cydia — it’s now at version 1.2.1. If you’re not already at