I don't think that this is related to HT, because i got the same
problem on an amd64X2.
It happens since nvidia-drivers >100.14.xx for me and is reproducable.
Run a kde-session, then start another kde-session (owned by another(?) user).
Some minutes later (when using something like flash in a browser or so) session :0 just crashes(back to kdm) while session :1 stays alive.
As my 2 pc are multiuser machines, every ">nvidia-driver-100.14.xx" is totally unuseable.
There are also some threads that seem to be related to this problem
PS: Dear guys @nvidia, could you please create a bugzilla or something like that?
Reporting bugs in this forum is not really helpful, actually nobody
knows if somebody did already file a bug or whatever.
Additionally nobody ever really knows if bugs got fixed.
Thanks for reading