Returning multiple rows in functions

I'm missing something in the docs - how can I write a function
that returns multiple rows? I want to do the equivalent of:
   select name from user_category where id = 21;

I create the following function:
   create function getid (int4) returns text as 'select name from mytable
where id = $1;' language 'sql';

... but it only returns the first matching row. Is there
a way to return all the matching rows?

Thanks,
Kief