I've been thinking about this too (doesn't everyone).
When it comes to ethernet shield interraction I've found it easier to have the arduino poll a web site. I've got a local web server (I have a QNAP NAS box that does web server as well as being NAS), and my arduino polls a website for instructions every time it's idle. The web server is built to handle multiple threads, whereas my arduino is going to get confused if multiple requests come in at once.
I've also thought about having an arduino per room or per pair of rooms, so I'm not trying to do too much on one arduino and it'll simplify the wiring.
I got myself a couple of smart card sockets and ten smart cards so I can do door access. I'm renting at the moment though, but I want to buy mainly so I can do stuff to my house that I can't when renting.