Daniel told you last night that the official PTR boards were up and character copy was live...but no actual PTR servers were active. We've known a 2.4.3 was going to exist for a little while, since Iriel mentioned it in the UI forums. But, the mystery of when seems to be over at least. Hortus announced on the PTR forum that the 2.4.3 PTR is on its way, and the forums just happen to be up a little early. It's coming fairly soon, too -- he's already telling us to delete our old WoWtest folder in preparation.
2.4.3 seems likely to be built of minor bug-fixes. We've not seen any of the doom and gloom that tends to surround class balance. It's definitely feeling to me like Blizzard's taking the time and effort to shore up their services as they prepare for Wrath of the Lich King. Still, let's see what 2.4.3 has for us.