CTSamurai Forums

It seems it's not just the game itself that's changing in Cataclysm. The launcher and background downloader now allow you to play the game before the entire patch has been downloaded and installed. It will continue to download and install the latest patches while you play, though obviously you won't be able to access content that has yet to be downloaded.

While they're calling this the Cataclysm Beta Launcher, I'm assuming their plan is to make this feature available for the retail release as well.

More information here and here.