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.