I just bought a book called 'Symbian C++ for Mobile Phones' and that has a long tutorial on writing a two-player battleship game. I'm trying to read the book in order so I haven't got to that bit yet, just noticed it when I was skimming.
has some (messy) bluetooth multi-player networking code..
(i can provide you with a newer release including tcp&bt networking if you are interested. i simply didn't find the time to update the site, yet.. :-( so the release available there is a bit out-dated.. :-)