To work around with the unlock issues on ios 4.3.1. Unlock your iPhone 3gs, 3g on 4.3.1 if you have an old baseband.
1. Download UltraSn0w 1.2 from Cydia (Source: repo666.ultrasn0w.com)
2. Download ultraSn0w fixer for 4.3.1 from Cydia (Source: djayb6.com)
If you are having any other issues. Please let me know in the comments section.