As of the latest carrier settings update the Personal Hotspot option is no longer usable unless you pay $15/Mo for tethering. That's where TetherMe comes in. They also use user agent detection however it doesn't block anything but default Firefox/IE/Chrome desktop user agents. There are easy ways to change your user agent with Firefox/Chrome. Most other applications work fine. The other way they detect is by using a special APN for tethering. That's easy to change.
2 Hoppers w/Sling / 2 Hopper 2000s / 1 Joey
1 ViP722k (separate account)