"cross-platform networking between all three platforms is not supported"
The first thing came to mind with Zune XNA was to be able to use it as a controller for my 360 ie in a Adventure game where you would select questions and inventory using the touch pad on the Zune that would relay to the 360. That it could be used allow the game to be played insync with both then desynced alone. So your at home playing the game you sync the game with both Devices, leave for work on the train you can continue to play on the zune get home again and sync up the game continue on both
So this idea is impossible then?.