For my PBBG Engine I am developing a command processor independent of the client interface. The reason for this is that I want to be able to build different interfaces such as a mobile interface, silverlight and/or flash interface. I want all the interfaces to process the same commands and be able to handle the same output from the server in order to update the UI components.

What I have come up with for the command request to the server is a simple pipe delimited string:

CMD|AUTHKEY

Where CMD would be the command to send to the server and the AUTHKEY would be a key generated when the player logs in through the authentication service that uniquely identifies the player for that session.

The response from the server will be JSON formatted objects and I still have not figured out the complete structure yet. The things I need to send back to the client would be:  An array of messages to display to the player, player's new position should they move, the player stats when in combat, their current target and any other players or mobiles they encounter. Could end up being a large set of data for certain actions so we'll see about performance there.