I think its software related. My 4,6 and 0 buttons just stopped working. Turned phone off and back on and now they're fine... Getting this quite a bit with this phone -
freezing completely (had to take battery out to switch it off),
not seeing SD card - restart fixes for a week or two
I always download the latest software updates - so my guess is shoddy testing by sony programmers.
JerryQ, September 2009