It's an extension of the trend for sites to have all kinds of modal popups for new users, or to alert users to new features, that are shown once or a few times and then stop. It's probably a good idea in general, though I don't like them if I have to dismiss them manually. At least please make them go away if I hit ESC.

It might be a cool thing if a site prompted the user at some point "You've been using our system for a little while now, would you like to try a denser/more compact layout?"