Drysc gave a little insight into how the Arena seasons are scheduled over on the forums -- I had always thought that the Arenas were running on their own schedule (with the spaces in between augmented by when patches got released). But apparently the timing of Arena seasons has more to do with PvE content than anything else.
Strange, no? But true -- Drysc said that they want Sunwell Plateau gear to "circulate" a bit more before introducing a new Arena season. That means that the players running PvE content are actually calling the shots for when the Arena season ends -- if guilds take their time getting through the Sunwell raid, it may be a long time before we see any signs of Arena season 4.
That's interesting -- obviously a lot of players consider PvE and PvP to be completely different parts of the game, but not so with Blizzard. Apparently they consider them very much two sides of the same player base, so much so that even something as arbitrary as PvE gear distribution determines timing of the Arena seasons.