STABLE has no effect on PL/pgsql functions

I noted in my PG 7.4 on Debian Sarge the following behavior:

EXPLAIN ANALYZE SELECT *,reallyHeavyFunctionDefinedAsImmutable() FROM
my_table;
time taken: forever

EXPLAIN ANALYZE SELECT *,
(SELECT reallyHeavyFunctionDefinedAsImmutable())
FROM my_table;
time taken: not very long

First question: Is there a good reason for this difference ?

Second question: If not, when will it be (or was it) fixed ?

Netzach