apple has consolidated their product line. there are only three notebooks left: 13" MacBook, 15"Macbook Pro and 17"Macbook Pro.
So the old difference between the two cheap iBooks and the three expensive professional Powerbooks doesn't really exist anymore.
Also the Mac's get compared to PC's of similar price/spec. And many PC makers will switch to Merom immediately or lower their prices a lot (because merom is a better chip for the same price).
Therefore i guess they will update them at the same time, give or take a couple of weeks to clear the distribution channels and get production running.