Why do you want rid of the jailbreak?, are you having trouble with it?
If it's activating problems you have, put into recovery mode and run redsnow.
I had problems a while ago with this and from memory I ran Blackra1n first that failed but did leave the phone in recovery mode. Connected to itunes and selected the Restore & Update version (to 3.1.3), then ran Redsnow 0.9.4 with the BL-39 and BL-46 files to jailbreak and unlock in one go. It worked fine and that US phone is now working in Sweden with a local sim.
ps, although the phone sw was updated to 3.1.3, you MUST select the ipsw 3.1.2 file while running redsnow.