Taken from, extremetech
"A mock-up demo was given to show how this will all work. Basically, you can be logged into Live with your same single Gamertag on the 360, on a Vista PC, or on a Windows Mobile phone. Anywhere you are, you'll be able to see your friends list and what the others are doing (if they want to let you), just like you can on the 360 today. Text and voice messages, game invites, it all works cross-platform. Xbox Live Arcade even exists across all three, and if you buy a Live Arcade game once, you can play it anywhere. For instance, if you purchase Zuma for 500 marketplace points on the 360, you can then play Zuma on your mobile phone or Vista PC (assuming Zuma has been ported to both).
Live on Vista
click on image for full view
Live Anywhere could very well be the killer app that ties together Microsoft's console, PC, and mobile businesses. Console gamers already acknowledge that the Live service is the 360's best feature, and making it work seamlessly across the PC, console, and mobile space is perhaps the smartest thing Microsoft has ever done."
