Does iscachable work?

Hello,

I'm running PostgreSQL 7.1.3, and I have a query that uses one of my
functions:

select findregion(entityid) from msg200;

The findregion function is set to be 'with (iscachable)'.  However, the
query takes upwards of 40 seconds to run, consistently.  The table only
has 20000 rows or so.  It doesn't seem like the caching is working.

The only other time I tried to use function results caching, it actually
consistently increased my execution time.  Anything I need to be doing
differently?

Thanks,
Fran