quickly getting the top N rows

If I have this:

create table foo (bar int primary key);

...then in my ideal world, Postgres would be able to use that index on bar
to help me with this:

select bar from foo order by bar desc limit 20;

But in my experience, PG8.2 is doing a full table scan on foo, then
sorting it, then doing the limit. I have a more complex primary key, but I
was hoping the same concept would still apply. Am I doing something wrong,
or just expecting something that doesn't exist?