The HP Slate 500 has been working flawlessly after the fix of the WiFi problem, till this morning. What happened was called as "ghost tap" by many users - the screen behaves like being touched by nobody in a random pattern. Watch this video for what it looks like:
The workaround is to do a n-trig calibration. You may run the NtrigMTMCalib.exe found in the ntrig install folder (usually in C:\Program Files\N-trig\N-trig Software Bundle), or start this utility in the ntrig panel.
[update] Happened again this morning (2/16/2012) and cured by a calibration. The 500 had been in hibernation for 15 hours when I woke it up. Could it be the n-trig driver does not initialize properly after a hibernation? Anyway, if this "ghost tap" happens too often, I'll write a script to do calibration automatically whenever the 500 wakes up from hibernation.
[update] So far as of 2/22/2012 I have not seen this problem again, I have been checking it at least every morning.