I've tried half a dozen times now, and every time I try to turn tethering on I get a message: "MyWi was not able to enable. Rebooting may help MyWi re-enable. Would you like to reboot your iPhone now? (Reboot) (Later)"
Edit: Per the ModMyi forums: SOLUTION: General > Reset > Reset Network Settings.
Edit: Per the ModMyi forums: SOLUTION: General > Reset > Reset Network Settings.