If I may "throw my hat into the ring" here....
After all the rumors I've read, I'm starting to get this "big picture" of what the update scenario might be:
I can see Apple doing an update "across the board" primarily because they'll release new Cinema displays with HDCP video connectors on them. That means none of the current models of Macs will be able to connect to them. Revising the Macbook and Macbook Pro notebooks with newer video chipsets and HDCP connectors for external displays means they'd need to do at least a slight case redesign anyway, to accommodate the different size holes needed for the connector change. So they may as well go "all out" and give us something totally new and fresh looking while they're at it, there. Obviously, newer mobile CPU choices are available now, as well, so I'm sure we'll see those put to use too.
In the case of the Mac Pro, a revision will probably mainly consist of offering Blu-Ray drives in the machines (again, to pair nicely with native HD resolution Cinema displays), and new video card offerings (maybe the new ATI 4850/4870 they claim they've got coming soon for Mac?).
In the case of the iMac, it might be possible to attach a newer, HDCP connector enabled Cinema display to existing models as a 2nd. display option, by simply selling a required adapter cable/box that plugs into the "mini DVI" connection they've already got on the sides of them. That means they won't be as pressed to quickly update/refresh them ... and a later refresh could be as easy as a speed-bump.
When you look at it this way, it doesn't really seem like Apple's "outpacing themselves" too badly with this whole update. They always do the iPod refreshes around the time-frame predicted for this new updates, too - so that may well have been designed and ready a long time ago.... It can't be that hard to change around the shape of the Nano casing and select a different format of LCD screen for it each year.
I don't know about all this. Seems like they'd be going hog-wild with hardware updates. After the MobileMess and CrappStore, doesn't it seem more likely Apple will pace themselves? There are some educated predictions, but timing is certainly suspect. I mean, really, a huge refresh of the iPod and Mac lines (less only mini, xserve and pro) all at once...
I guess we'll see.