UPDATE
I agree with Dave (see his comment) but only to a point.

Using a whitelist is a great idea. Unfortunately it is not always implemented well. If you only allow emails from your whitelist you will be missing a lot of good mail. And this is a problem for more then just a business, as Dave points out. I have the same problem with whitelisting as I do with challenge-response services. They block legit email. What about the email confirmation for that plane ticket you just bought? I could come up with a number of other "what ifs" that whitelisting and challenge-response services would block.

Apple's junk mail filter (if you use it) does have options to exempt address book entries from the junk filter. SpamSieve has this same type of option. POPFile uses something called "Buckets" which is a whitelist and a blacklist, but does the same as Mail and SpamSieve. This is the way to use a whitelist. You should not except ONLY the addresses in your whitelist.

Going further here, I don't like making addresses in my address book exempt from my whitelist. This is especially true if you are a Windows user, as many security threats on Windows reck their havoc using your address book. But Mac users have to deal with this as well, mainly because of those Windows born viruses and trojans. A few months ago a couple viruses went through our university email system, and I "got" so much mail from BGSU contacts in my address book. Of course it was all spam mail generated from a Windows born virus. So I do not include my address book in my whitelists.