I purchased a T-bolt on release day and recently started having issues with the proximity sensor not turning off the screen during calls. I troubleshooted it by testing the auto-brightness feature, which did not work as well.
I found the fix online in a thread about the HTC Desire.
Apparently, the gap between the top of the screen and the earpiece (near the proximity sensor) allows dust in, which causes the sensor to not work.
You fix this by simply blowing air into the gap. I did battery pulls, reboots, and installed the "Sanity" app to try and fix the sensor/troubleshoot. None of this worked, but blowing into the gap instantly fixed it.
I hope this helps anyone having this same issue.