Yeah, I've seen that occupancy sensing solution, but unfortunately the limitation of it only working if the user unlocks his phone upon coming home is not acceptable to me. I'm actually in the process of writing my own geofencing app for iOS to ping a custom API that will solve this problem.
As for bluetooth:
While at home, I don't need any of our phones to pair to a second device, so I would be okay with that limitation. I do carry my phone around though, so sensing the vibration wouldn't work. Plus I'd like to use the data that a bluetooth device should be able to get, such as who is calling, and eventually work that in to my home theater system (to display who is calling in XBMC for example).
What's killing me is that I swear I read about some router-sized device with wifi and bluetooth (might have been an actual router, might have not been) that people were buying on eBay and whatnot for Home Automation because it was an excellent way to get a cheap computer with bluetooth capability. But I have no idea what it was!