Hey nobody liked the -- Apple delayed the death of OS 9 until June, so the legacy machines scattered throughout the lineup are most likely a marketing decision to keep OS 9 machines around until June -- answer.
Even though there is that marketing decision Apple could have released replacements early or late, and centered around June.
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Since the above answers didn't fit in with their view of the world, there have been rampant rumors -- then the 970 rumors started.
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If you don't like the above OS9 delay answer, you're really not going to like...
Hey, it looks like Apple is getting ready to EOL the last of the iMac/PowerBook/PowerMac legacy machines that boot OS 9, sorry the new machines will just be speedbumped machines.
But hey you might get FW800 in the DDR iMac and USB 2 in the Rev B PowerMac.
As for the PowerMac 970s, try Christmas*
* Told you that you wouldn't like it, it's just a possibility that will reveal itself and/or get buried in reality soon -- June/July.
And no --- the last part is not a guess, speculation or whatnot -- just another possibe way in which events could unfold.