Cisco are about to release an app that does this, but I don't think it will be cheap to implement. You need more than one AP and a layout of the building to include wall thickness, what they're made of etc.

The application then gets the information from all AP's about how strong your signal is to them, plots this on the map, factors in any walls etc and give you a position that allegedly is accurate to 3 meters.

I don't think it has been released yet but I could be wrong - last I heard was about 3 months ago and that it was still in development - it was called something like Cisco Location Appliance or something similar - a google will prob throw it up.