iRinger Tethering
The app is still (July 8) in the app store and does work, with some limitations. ATT iPhone 5 running 6.1.2. In order to get the app running, you do need to change the language to French, otherwise it will crash. I also could not use the URL generated by the app, but since you run a static IP address, I changed the URL to the IP address and that seemed to work just fine. Also as mentioned, you cannot background the app.
You can tether a tablet or iPad to the iPhone, sort of. As mentioned, these apps use the adhoc networking on a real computer to generate a network. However, if you have a small travel router, you can do the same thing without the need of a real computer. I have a portable, battery operated travel router that I can now share the iPhone data connection with multiple clients. All you need to do is to connect the iPhone with static IP to the router and then have the clients setup their http proxy to the correct address. Works pretty well. I can now connect a macbook, iPad and Nexus 7 tethered to the iPhone (fortunately I have unlimited data).
Web browsing seems to work just fine, but mail.app doesn't want to connect. Oh well, always can use the web client.
My needs for tethering is inconsistent and light, so the $1.99 for the app is worth it, even with the limitations. For me, the only additional thing to tote along is the small travel router, and once the iPhone and clients are set up, then they only need to connect to the travel router with no additional configuration.