As usual this was another one which took forever to find. After the Spirit Jailbreak for iPhone OS 3.1.3 was out, everyone started searching for the tethering options. The SinFull "Enable Tethering" via Cydia didn't work anymore since they were compatible only with 3.1.2. The only option was to go via MyWi App available on CYDIA. The new feature on MyWi is to enable tether via WiFi as well as USB. Now MyWi also has an option called "Auto Fix Tethering". Try that and it will bring back the Tether Option under Settings > General > Network and will enable it by default. Now you don't even have to start MyWi to have tethering. You can download MyWi from Sinfuliphone Repo after adding it to CYDIA.