I used to have the G1 data plan (unlimited data + 400 text) for $24.99. WHATEVER YOU DO, DON'T CANCEL THAT PLAN. That's the true unlimited plan that they don't put any throttle or soft cap. I regret is so much I changed mine to a family plan.. with the 2GB soft cap, it goes in a few days. Now I called back to see if they can switch me back.. they can't as it is not listed in the system anymore.
BTW, this is the phone's problem. I have both Sensation and S2. I can still do WIFI tethering in Sensation but not the S2.
Also, once you have the WIFI hotspot turn on, your phone won't go online also. Browser will redirect you to the same "stay connected" up sell page.