I guess if you don't mind filtering through your "spamfiltered" mail folder every other day to find false positives, the whitelist-only thing is an okay idea.

I've been running SpamAssassin (www.spamassassin.org) for a few months now. Great piece of software. I get about 40-50 spams a day (plus countless ones in non-English character sets, that go straight to /dev/null) and on average 2 of them make it into my inbox. I still have to sift through the spamfiltered stuff every couple days but it's very quick since there are so few false positives after a little tweaking.