Facebook, despite the blogosphere's perpetual bickering, actually has good, granular privacy controls. You can control exactly who can see each section of your profile, and easily block specific people from even finding your profile.
One lesser-known feature, however, is that you can control who can read each and every one of your status updates. You've probably seen the padblock below the status update box, but if you click it, and then Customize, you can block specific people from reading your update!
Maybe you're not quite ready to unfriend someone, or perhaps you're simply trying to arrange a surprise birthday party for one of your friends -- either way, it's a great little feature. As a commenter points out, it's also rather handy if you want to block family members from reading... sensitive... things.
Now, if only you could control who could read your posts on your friends' pages...