I'm in the UK. When using Pictochat, my American system doesn't show as appearing in any of the chat rooms... on the list of chatrooms they sometimes list as having "0" people in them, when in fact they do. We're all in there really and it still works. In fact I've seen this glitch happen between two European systems and it doesn't happen all the time, so maybe its just a glitch.
When trying to use international software (ie. EU vs US Metroid Hunters demo) it just doesn't work. This wouldn't necessarily have to be the case if the developer accomodated for it, but Metroid seems like a very early demo indeed that probably saw some changes between versions... hence the lack of compatability. Single cart download multiplayers like Mario 64 work perfectly.
In local LAN games that need multiple game cards its the safest bet to make sure you have the SAME software as your friends.
However I don't see this being the case with the online games. I have a feeling that if they truly want people to compete worldwide (if they so choose) they will make sure each version is inter-operable. Mario Kart for instance is seeing a near simultaneous worldwide release (or as close as you're going to get)... I've got a feeling the only differences in the software there will be the languages. No other tweaking.
Hardware is irrelevant other than the weird glitch in Pictochat I mentioned. It plays all software.
