You can try FoxFi as Sydman said, or you can hack the built in hotspot app fairly easily. Go to the market and search for SQLite Editor. Its like 2.99 but what your doing can be done in a couple minutes then you could theoretically do a refund. But get the app, open it up. Superuser is going to ask for permission, click allow. Now its going to load like 500+ items. Let it do its thing. Once its done, scroll down to "Settings Storage". There are 2 of them and you want the one with the hammer and wrench. Click that and now click on settings.db, then settings. Now go down to item #150, "Entitlement Check". Next to it you will see a "1". Long press on that field and and click "Edit Field". Change the "1" to a "0" and then click save. Now exit and reboot. After your back up and running go to your mobile hotspot app and click "Turn on Mobile Hotspot". Voila, tethering restored!