SQL_BIG_RESULT being ignored?

[kabel]

Using 5.0.51, I have a fairly substantial SELECT ... GROUP BY query to
which I have added the "SQL_BIG_RESULT" hint.  

According to the 5.0 SELECT manual,
"SQL_BIG_RESULT can be used with GROUP BY or DISTINCT to tell the
optimizer that the result set has many rows. In this case, MySQL
directly uses disk-based temporary tables if needed, and prefers sorting
to using a temporary table with a key on the GROUP BY elements."

However, during query execution, I saw "Converting HEAP to MyISAM" in
the processlist for that thread.  I may be misunderstanding the manual,
but shouldn't that hint force the temporary table to disk rather than a
HEAP table?

Thanks for any help anyone can provide,

kabel