Apple has made clear that it was expanding its iOS 5 notifications to the desktop with OS X Mountain Lion, and now it looks like they are extending it to the web browser too. Some visitors to iCloud.com have reported seeing notifications show up at the top of the page that were both based on the iOS look and which were clearly not ready for prime time.
Did they accidentally tease it? There's no indication of how advanced notifications might become in Apple's web portal, or if they'll transition from development cocoon to public butterfly at all. But rest assure, we might just be seeing that in the near future.