Domain types versus anyelement

I'm getting some surprising behaviour from domain types in 8.3.7.

I have a domain "aud" on numeric(20,2). When I try to use a value of
this type with the built-in greater() function, I get this:

# select greater(1::aud, 0);
ERROR: function greater(aud, integer) does not exist
LINE 1: select greater(1::aud, 0);
^
HINT: No function matches the given name and argument types. You
might need to add explicit type casts.

I see that the argument types for greater() are (anyelement, anyelement).

It seems peculiar at best, that a domain on a perfectly valid
anyelement type is not considered a candidate match for anyelement.

Cheers,
BJ