The MacPro is going to drift to be a similar update cycle over time because they are going to be similar in quantities sold and targeted markets. Seriously, what is wrong with a server/pro class machine coming out once every year ?
Nothing wrong with that, but why not adjust ram and hd (or gpu) configurations mid-cycle to current standards? After all prices for these parts go down through a long life-cycle and it's hard to justify margins rising along with the age of a product, as perishable as modern electronics are?