SQL function problem in 7.1?

Am I doing something stupid with strings in SQL functions.

richardh=> select version();
version
------------------------------------------------------------------------
PostgreSQL 7.1beta3 on i586-pc-linux-gnu, compiled by GCC egcs-2.91.66
(1 row)

richardh=> select * from foo;
a | b
---+-----
1 | aaa
2 | bbb
3 | ccc
(3 rows)

richardh=> create function fooa(int) returns text as 'select b from foo
where a = $1;' language 'sql';
CREATE
richardh=> select fooa(2);
fooa
------
bbb
(1 row)

richardh=> create function foob(text) returns int as 'select a from foo
where b = ''$1'';' language 'sql';
CREATE
richardh=> select foob('bbb');
foob
------

(1 row)

TIA

- Richard Huxton