I've now been working here and there when I've had the time with this project for a while. I have migrated from bluetooth to wifi, and I've put the "talking to a visitor" part of the project on hold, as many hours of research deemed it too hard to finish with my current level of Android/Arduino audio experience.

I purchased a WiFly RN-171 (that I had some problems with in the beginning, works great now) for adding wifi to the project. It now connects to my home network, and as I have coverage outside my building I can actually use it to get inside as well as opening the door from my couch ;)

The Arduino and Android coding is pretty much finished by now so code, pictures, details and explanations will be added soon!