'-U postgres' doesn't authenticate

I'm using PG-7.4.2 on a SUSE-9.1 box. The docs say that I need to be the
'postgres' user in order to do things like create databases, which makes
sense. If I switch to that user (using 'su') it works fine, but if I use
the -U switch from my own Linux account I get an error:

 >createdb -U postgres test
 createdb: could not connect to database template1: FATAL:  IDENT
 authentication failed for user "postgres"

My understanding is that this should work, and it would be a useful shortcut
for me if it did. Do I misunderstand or have SUSE set something in the
config to disable this feature?