I had the same problem, but it
disappeared when I upgraded xorg (and/or some other X stuff) which was
uploaded a few days before or after the xlockmore upload.
Something with DPMS is still not right though:
when locking, it switches the backlight off and on, and then again off and
on when unlocking. Went back to the old 1:5.27-1 (had to download it from
ubuntu!).
