The big announcement has been made, and it's about what one might have expected. Season 5 for the Arena will begin on December 16th, when level 80 characters will begin rated play to compete for gear and end-of-season rewards.
This announcement came during a reminder that the "Limbo Season" will end this Tuesday. That means you're not going to get Arena points for any rated matches you're playing this week. There will be no rated Arenas again until December 16th, which is kind of a shame when you consider the two new Arenas in place.
A side effect of this announcement, of course, is that we now know for certain when Blizzard expects most of their players to reach level 80. Since you have to be 80 to get into the rated matches, you can guess the developers were waiting for a significant portion of the playerbase to be ready before starting the season. No doubt, high end Arena fighters will already have been storming the battlegrounds and collecting honor before the season starts.
WIth the gear changes and mechanics changes, it's going to be exciting to see the first season of Wrath. I'm definitely excited to see what new dynamic Death Knights bring. It's certainly going to be a different show. Good luck!