Some previous discussion:

http://news.ycombinator.com/item?id=924237

http://news.ycombinator.com/item?id=927299

I wonder how easy a similar set-up with a Raspberry Pi would be. The option to use the same hardware, but provide a web service to control the door as well, or other authentication methods would be cool to see.

Apparently I missed the original as well! - it's at:

http://grathio.com/2009/11/secret-knock-detecting-door-lock....