Why don't you wait a few days for the 3GS jailbreak tools for 3.1 to be released.
If you have an error JB that 3GS, you will have to update to 3.1 to get it working again, and doing so locks your baseband so the phone can't be unlocked again.
Google "DevTeam blog" for info on 3GS JB for 3.1 firmware.