Okay, here goes, only my somewhat educated opinions though
Portable chips for portable computers.
iMac G5 was a desktop chip, now a portable chip in the intel iMac, maybe that's the way they want to go, but I'm guessing it's because they wanted an intel machine out there fast so that lots of people would grab onto it initially and easy the transition, and it has seemed to work.
eMac is being EOL, eMac are generally the old iMac, new eMac said to be released soon so goes in line with new iMac design. Since iMac was first to make transition, would make sense that it's the first (minus the MacBook) to be upgraded and put back to a level where it should be (desktop chip, Conroe, which I believe will be cheaper than Merom, so cost effective too), there, that gets Conroe in the iMacs
MacPros: when has apple not put the maximum chip into their top of the line Pro machine? I know people will say Apple has crippled a lot of their hardware, but would they cripple a MacPro to put a Conroe in instead of a Woodcrest? Unless Conroe is pretty much exactly the same as a Woodcrest, minus the multi-chip interoperability, and it sits in the same mobo, why would Apple want to have multiple, very different MacPros? Wouldn't that be a production nightmare? I would imagine that they want to have the best pro machines possible (to really stick it to PC) so 3 models, a single Woodcrest, a double Woodcrest (slow, 2.6GHz ?), a double Woodcrest (fast, 3.16GHz ?) something along those lines.
This also clearly delineates each product into its own class, based on the chip. The only overlap being the Mini with a Yonah, but those guys are pretty much portable anyways, and not a desktop.
Since the PowerMac is the last to be updated (minus the Xservers) and Woodcrest is the first chip out, it only makes sense that they will put the more powerful, earlier to arrive, chip in their top of the line computer.
These are my opinions, shoot them down if you want, but it makes sense to me.