I now see that others (Samsung, Windows 7) are having the same problem as me. My phantom data usage began this month with random huge data suck that is way out of my normal range. At one point, the phone transmitted - get this - 114 MB! More numerous are 20-60 MB. As I mentioned, I have not changed my phone use pattern and typical data transmits do not exceed say 6 MB with most in KB range. My monthly plan is 200 MB and have not exceeded it in 4 months of ownership. Neither ATT nor HTC seems to acknowledge this problem but these are at times when I don't even have the phone with me. I suspect I will need to wait until more people report this problem but it would be nice to have some help in the way of answers...help!
Have you ever tried turning off the data during periods of inactivity? The HTC phones have the ability to turn off the data connection very easily. At very least to test some of the times when you suspect data is transferred when you think it shouldn't be.