The 3.1 PTR is up and running. You can check out the patch notes and the WoW Insider Guide to Patch 3.1 for everything that is on there. Some of the big things you'll want to look at right away include dual specs, the argent tournament, and a host of new achievements.
Most of the WoW Insider staff is up late burning the midnight oil tonight, so we'll be pumping things out here as fast as we can. Should be an exciting day!