They announced that expansions were looking to be at a "1 per year" rate, but we know Blizzard has deadline problems.

I hear that, I've dropped tons of gold into my tanking gear/etc, not ready for it to be outdated quite yet.

Also, I don't think Blizz wants to make The Black Temple into the next Naxx by only having it out a short time before the next expansion hits to make it obsolete.