ISNULL performance tweaks

I have some existing code running in a production environment with 
embedded SELECTs whose WHERE's use ISNULL tests on indexed foreign key 
fields.  This is obviously very slow.

My ISNULL *queries *take anywhere from 6 to 40 seconds.  These queries 
are used to generate reports to a device which times out at 20 seconds, 
so half the time these devices don't get their reports, which makes my 
customers VERY angry.

I recall seeing an email (I believe on this list) about how to improve 
performance of ISNULL's with some sort of tweak or trick.  However, I 
can't find that email anywhere, and couldn't find it searching the 
maillist archives.

So, until I have the time to code the fixes I need to prevent the use of 
ISNULL, does anybody know how I can speed up this existing system?

Man, I wish PG indexed nulls!  Is there any plan on adding these in the 
future?

Thanks for any help you can give!

-- 
Matt Mello