gbattle:

Turntable.fm’s co-founder Billy Chasen has his business on lock, literally. You don’t use keys to enter Turntable.fm’s HQ, you send a text message from a white-listed phone number and voila, it opens. This is a much smarter, safer and cheaper option than handing out a ton of keys.

UPDATE: This won a Twilio prize and he explains how to build one yourself.