New 3.1.x compatible version of MyWi was just released today. I installed it on my JB 3.1.2 3GS and it brings back the Internet Tethering option under Settings > General > Network > Internet Tethering. Works just like it did before with 3.0!

EDIT: UNINSTALL PDANET PRIOR TO INSTALLING MYWI!!! I installed MyWi with PdaNet the first time and it bricked my JB. Phone wouldn't reboot--stuck on Apple logo. Had to do a full restore. Tried a second time without PdaNet installed and it worked perfectly.