Last Tuesday, I have started working on adding network-based multiplayer support to Tennix (a free 2D tennis game for Linux, Mac OS X, Windows and Maemo). In addition to two players playing on one computer (which already worked a long time ago), we now support playing Tennix over the network on two tablets (obviously, playing a tablet-versus-PC game will also work).
Currently, this network play runs over UDP, so WiFi connectivity is needed when you want to play tablet-versus-tablet. I might be able to add Bluetooth support later on, but the BlueZ API seems way more complicated than SDL_net's API which I currently use. Let me correct myself here: There's no real documentation for the BlueZ C API that I could find.
A small library with a clean API that abstracts Bluetooth, WiFi and maybe some other connectivity options and provides an easy way to establish a transport between two hosts/tablets would surely help boost developer interest in creating multi-player games and cross-tablet applications.
A video demoing the controls on two tablets can be found on YouTube.
