In case you were wondering...

You may have noticed the background downloader downloading something if you've logged onto the World of Warcraft client today. This download is a portion of Patch 2.1. Blizzard poster Drysc explained it as such:

The background downloader will grab bits of upcoming patches before they're officially released so that on patch day you only have to download a remaining megabyte or so, patch, and then you're able to log in pretty quickly. It's been a normal part of the patch downloading process for quite a few patches now, and it should have gone active yesterday I believe to start downloading part of the 2.1 patch. Actually the bulk of it more than likely as I don't believe it's too much larger than 250MB.

So make sure to log on to the game sometime so that you can get a portion of the patch!