While it's technically always been possible to stream (using the 360's MediaCenter Extender technology), the new update gives the masses who aren't yet using a MediaCenterbox a quick and easy way to stream video to the television.
As we all found out, however, users who prefer to roll their own MediaCenterbox (like yours truly) will be left out of the option entirely -- support will only be available for CableLabs certified boxen, which home-rolled are, of course, not.