I also thought of modular plugin based system. Here is a rough overview of how the components we may need.

Logic:
Game - the logic for a particular game, right now that would be just chess, but other board games could be possible too. (Chess based or other)
Pre-Game - How to find players, select a server or engine

Domain:
Board - Represents the data structure of the board (duh)
Player - information about the other player, like online status, skill level

Communication:
ICS
Bluetooth
Telepathy
Computer Chess Engine - an offline engine should be accessed the same way any online game is, the engine does not have to be in the same task