No way to do it over bluetooth, but it looks like you can do this over wifi using UDP multicast:
Windows Phone Peer-to-Peer Multiplayer Game using Sockets in XNA - Ricky Tan's blog - Site Home - MSDN Blogs
I don't think WP7 supports ad hoc networking. So unfortunately if you want to make things work without a network you'll have to wait for WP8One more question:
Is it necessary to be in the same wifi network or is a p2p connection between to WP devices possible without the need of a router? Because in your link it says: "Phones running the game on the same WiFi network automatically discover one another and the players just appear on the screen."