A Microsoft representative has confirmed with Joystiq "that the [cloud storage] service has not gone live yet. We'll keep you posted on launch details." Looks like we'll have to wait a while yet before access.
Original story: Microsoft have talked about adding cloud saves to Xbox Live, just like Steam and PSN do, to save us from chugging around harddrives, USB-sticks or consoles if we're going to play away from home. Now CVG reports that the service is actually online already, but only for a couple of Xbox Live Arcade-titles. We don't have a solid list of which titles have the feature, but hopefully we'll be able to save everything to the cloud in the near future.