I have tried Terrarium, but its P2P architecture doesn't really fit the model of a card game. My client/server model, with all logic localized to the server, seems to be a more appropriate architecture.
Also, I would assume that the Terrarium communication method is probably a .NET-based proprietary solution, and therefore unfit for creating connections between a .NET server and a Java client (which is the final goal of my connection scheme, as I want to be able to play this game with my Macintosh-using friends).
Kannan, thanks for the suggestion, but I don't think the approaches used for Terrarium really fit an online card game. Eric