Its bee brought up before. There's lots of ideas of how to do it. Here's my mind working through the process, and anyone with suggestions dont be shy.

• There's a lot of discussion about it: http://lists.hackerspaces.org/pipermail/discuss/2010-April/002074.html
• WhiteSpace has their GateKeeper project: http://0x20.be/Gatekeeper
• Hackerspace.lu has OpenDUINO: http://wiki.hackerspace.lu/wiki/OpenDuino# (also has a list of similar projects)
• We have... physical keys! \m/

So here's what walked through my mind as I'm reading this:

• Features wanted: Status of each door (open|closed,lock|!locked), control (User X gets door 1, Lock Door 1, etc), audit (User X unlocked door Y at 103939394).
• Simple, cheap, requires little maintenance.
• Requires little to no extra hardware
• Solenoids for door locks?
• Manual override (just in case)
• Not arduino.
• Not specific to us. Simple, extendable.
• Cerberus-Prox?
• Keys/PIN
• RFID or No RFID? (Adric/Aaron, I'm looking at you)
• Alarm? I have a fairly loud alarm siren. Could be success.
• Door Status -> magnetic switches.
• Database should be self-contained (I dont like MySQL, but like SQLite)

Not much time to discuss right now, but though id drop a few tidbits here.
we have atleast 3 paralax readers, http://www.parallax.com/tabid/768/ProductID/114/Default.aspx they interface nicely with arduino, and or anything else you can talk ttl.

and a dozen keyfobs/cards.

we also have a few pinball solenoids.

problem i found when thinking about implementing it, is the complexity given our current structure, 2 knoblocks and 2 deadbolts, one each door, anything less and we are possibly inviting theft/tampering.

most systems involve either a solenoid strike retracting, or a motorized deadbolt/knobset or a maglock like jetro is suggesting.

to do a solenoid strike, they only work well for knobsets not for deadbolts, one owuld have to cut a section of the door frame.

motorized deadbolts/knobsets are not cheap,

magnalocks require a fairly sturdy non bendable door. and strong anchor points. (also require some though on fire-escapability, and also power interruptions..

let me say im all for us hacking together something, but i think the ultimate goal should be to not decrease security, cause un-reparable building damage, and a requirement of high reliability.