That doesn’t seem too hard, and quite inventive! Here’s how you’d do it, in my opinion:

Hookup an arduino to the intercom system similar to my instructions
Setup the arduino to hook to the web (http://arduino.cc/en/Tutorial/WebServer)
setup a webmail account to send to when you want to open the lock
Setup a web service that will open up the mail to the mail address you setup, and setup a chron job to call it every minute / second (http://stackoverflow.com/questions/114953/how-to-get-email-and-their-attachments-from-php)
Setup the arduino to run the script. If the mailbox is full, delete all mail, and trigger the arduino to open the door

So you can send an e-mail from your phone to an address and within a minute, your door will open!

What do you think?

Dom