In OS X Mavericks, websites will be able to send you push notifications in Notification Center. This means, for example, that every time Cult of Mac publishes a new article, you could get a notification through Mavericks.
Although both sites have prepared their push notification support, they have yet to send out any tweets.
Apple tends to give early notice about upcoming launches to big industry partners like The New York Times and the NBA. My guess is if they’re going live with this, they’ve probably been tipped off that OS X Mavericks will be released today. And with the GM version available for some time now, they may very well be right.