We were all taken by surprise this afternoon when we heard the PTR has gone live for Patch 3.1.2, because we had little indication that it was going to have a public testing phase at all. It makes some sense looking at the content of the patch, because it seems they're trying to implement the Equipment Manager again, rather than waiting for the next full blown content patch. This patch also contains the new round of Argent Tournament mounts described a short while ago.
When full content patches hit the PTR, the developers generally do not allow premade characters to be created so it doesn't taint the data gathered from dungeon and raid testing. However, this PTR period doesn't have any content like that, so they are allowing premades. If you've been wondering how X or Y class plays at 80, this is a good chance to play around a little bit.