Intel isn't releasing the chips that are 64bit that Apple wants to use for a couple more months. Intel just highlighted the chips at a conference this week that will be used in both laptops and future PowerMacs... or whatever the pro macs will be called.
Also, 64 bit chips are generally hotter running and require more power. So even though Apple is going 64 bit in many places, 32 bit makes sense in many places still.
So far they've replaced the Powerbook, Mac Mini, and iMac.
Of those, only the iMac was a 64-bit computer, previously. Given that it could only take 2.5 GB of RAM before and is a consumer-targeted computer, I can't think of a single reason why it needs to be 64-bit. Not even Final Cut Pro gets any advantage from a 64-bit computer vs. a 32-bit. It's most useful for some science applications and super-computer farms, neither of which tend to use iMacs.
I am sure that the Intel Powermacs will continue to have 64-bit chips. The iMac never needed them in the first place...it's just that they only had one version of the G5 chip so that's what they got.