The 'base' hook in mkinitcpio does not create /dev/mem .
This might not usually be a problem, but I recently ran into
a problem because /dev/mem was not present at boot.

The problem in question arose while attempting to use
uvesafb (an excellent framebuffer driver; helped a lot
with my laptop with intel graphics). It wasn't able to
query vbe 3.0 support without /dev/mem existing...

The fix was easy; I created a 'mem' hook that made /dev/mem
on boot :)

Of course, the best way is probably for it to be created by
the 'base' hook, which creates other device nodes like /dev/zero.