Blizzard have confirmed that the background download we are starting to encounter is the pre-launch Pandaria data. Looking at the download screen, it is estimated that initially it is set to download an extra 6Gb of data (its states 21Gb, but quickly jumps past the first 15Gb, suggesting most of it is verifying current data).
No doubt there will be several additions to that as we get closer to launch date.
There are various rumours as to when the pre-launch data will actually go live - some suggest August 28th, to coincide with Guild Wars 2 launch, others suggest a week either side. I'm sure we'll find out soon enough