Before making /dev/mem world-readable (which would solve your present
problem, but which would not be good for security :-)), I'd check
which group owns /dev/mem, and the permissions on /bin/ps.

/dev/mem should be group kmem, group readable, but NOT others
readable. /bin/ps should be group kmem and setgid. That way,
/bin/ps can read /dev/mem, but other things can't.

Also, why is it choking on /dev/mem and not /dev/kmem? If I'm
not mistaken, ps would have a lot of use for /dev/kmem but none
for /dev/mem...

For reference, here's an ls on my system:
144 -r-xr-sr-x  1 bin   kmem    139264 Oct 19 15:05 /bin/ps
  0 crw-r-----  1 root  kmem    2,   1 Nov 10 06:49 /dev/kmem
  0 crw-r-----  1 root  kmem    2,   0 Nov 10 06:49 /dev/mem

Good luck!
	-Ben