Ordinn from Blizzard posted the following information today regarding the move of many servers to new hardware.

First off, we’d like to thank everyone who helped us test our new hardware on the public test realms over the last month. The testing allowed us to move forward ahead of schedule with the improvements outlined at the beginning of March: http://forums.worldofwarcraft.com/thread.aspx?fn=blizzard-archive&t=45&p=1&tmp=1#post45. We were able to bring the new World of Warcraft site live more than a week early, which in turn allowed us to open 12 new realms over the last two weeks.

The next step in our ongoing efforts to increase game performance is to migrate a set of realms to the new site this Thursday, April 6, beginning at 12:01 AM PST. The migration could take up to 24 hours to complete, so we will be crediting players with active accounts who had characters on these realms as of 12:01 PM Monday, April 3. The realms that will be affected are listed below.

Check out all the details and servers involved here.