+ARENA_MASTER_NO_SEASON_TEXT = "Arena battles and Rated Battlegrounds are only available during an active player versus player combat season. Season nine will begin on December 14th, so be prepared at that time to take your shot at glory!";
That entry indicates that the next arena season will start on December 14.
Combined with a recent official blue post on the World of Warcraft forums stating that the next arena season would start a week after Cataclysm's release, that seems to indicate a release date on or around December 7.
While I have every faith that the blue post from the forums was accurate, I find the patch notes entry a little sketchy. Patch notes change all the time, and this particular entry could simply be a placeholder.
We dropped Blizzard a line to ask about the date, and received the expected response.
"We haven't announced a release date for World of Warcraft: Cataclysm and we do not comment on rumors and speculation."
All will likely be revealed come October 22, when BlizzCon 2010 kicks off in Anaheim, California.