did something similar in school a couple years ago. Basically was a web server and an arduino connected to the apartments buzzer and door unlock system. When you were at the front door you would be connected to our wifi network and could then go to a url that would open the door. Never thought to sell the service to others in the building - would have been a good idea. although when the maintenance guy came in and saw an arduino hanging down from the intercom/buzzer box he was a little pissed. lol