I’ve seen this happen a few times on my HTC Desire. I’ve never figured out whether the problem is in adb or the device but I usually do the following:
- Restart adb by issuing ‘adb kill-server’ followed by ‘adb start-server’ at a cmd prompt
- Disable and re-enable USB debugging on the phone
- Rebooting the phone if it still doesn’t work.
99% of my issues have been resolved with these steps.