Steve and Apple have Mac users figured out.
They excel in creating hype, and use panic to their advantage.
I think they learned this from the Segway guy. There was this new thing, it's going to change the world, etc. It turned out to be a stupid electric rolling platform that only lame mall-cops use, as far as I can tell.
Steve has done the same thing. He hypes up new products that will "change the world as we know it," and it is always just some cute junk that people have convinced themselves they HAVE to have NOW. It's no different with the "pro" line.
By keeping silent about the newest pro machine, an environment creates itself in which people speculate. We've been trained into a Pavlovian response in which we demand being constantly fed new products. Enter the iPad, and see the feeding frenzy it created. There is only so far that stupid thing can go before the lipstick wears off, but it will hold us over long enough for a new tower to be polished up.
In the meantime, old Macs keep getting older, and the pool of users that feel their machines need updating grows larger. Once it reaches the breaking point... BOOM! Steve hypes up the NEW MAC PRO that kicks the ass of the Sun God Ra himself, and we will tear out the eyes of children to get it after waiting so long.
See, Apple is waiting. This tech wave with i7 and beyond-quad-core was one of the smaller waves, and they're waiting for the bigger waves behind it to come, so they can finish bigger with more style.
In my opinion, there is evidence of this in the cycle / waves of Android vs. iPhones. The 3G and 3GS were on top, then they let the android market swoop in behind them, but on a smaller wave at first. Android started to pick up and fill out, getting cooler, more hip, more widespread... and Apple throws out the iPad-iPhone 4 one-two punch. It's all just a little bit better than Android - - better screen resolution, front and rear cameras, multi-tasking (finally) and powerful enough to edit HD video on. (Edit video on a stupid phone, really? That's another rant for another time.) Now they're on top again in the mobile market, and it will take that much more effort for their competitors to out-surf them in the next heat.
Apple will build a new Mac Pro, and it will be scrumtrulescent. The pros will suck from the teats of it, savoring the nectar it gives with crack-like addiction, and have no problem justifying the increased cost, because it's the top of the big wave again.
I jumped on this train right as the 3.33GHz model came out, and I'm happy. I think it will last long enough to allow me to skip one wave (Mac Pro 5,1) and jump on the following Mac Pro 6,1 wave. I don't believe pros will be editing feature films on phones in five, ten or even twenty years, but things will change for sure.
Why sweat it? Go with the flow, mang.
