Hi
SD would be good or maybe an ethernet shield, external logging and access. Maybe some kind of remote locking/unlocking.
Someone rings you up cos they don't have a card, you then open the door remotely. Maybe have a network webcam set up on the control page too.
i've done a couple of project like this:
Debian Lenny 
- ZoneMinder (web OSS surveillance app - http://www.zoneminder.com/)
- ip cam (one or more, of course)
- Arduino 2009
- Arduino Ethernet Shield
- relay board
- jQuery (for asynch. GET/POST request to Arduino Eth. Shield)
Here's a sample screenshot (remote command buttons are in italian lang)
You can use also Motion (another OSS surveillance app), that is quite simpler than ZoneMinder.
Regards
M