The last decline in PC shipments was a 5.1 decrease back in 2001 due to the crazy dot com bubble, so it's not like PC sales decline regularly. That makes this 4% drop quite unusual.
What does this mean? Most likely, it means we were right: the desktop PC is sputtering out. Not all the decrease can be attributed to the weak economy, seeing as notebook PC shipments raised by 11.7%. Pretty soon we're going to see desktops become a niche market, reserved only for pros, dental assistants and gamers.