64bit is not important in an imac. 64bit is actually really not that important at all unless you want more than 4GB of ram. Although on x86, 64bit has a nicer tweaked instruction set, but yonah will still be the best x86 CPU out around early 2006.
I find it unlikely though that apple would put a dual core CPU into a consumer product like the imac. There will be a single core yonah. There is no reason to place laptops at a lower level than desktops, the powerbook is a pro product and ideally therefore should perform better than most desktops. If not for the slow G4 it has to use now it would be faster than the imac also. This is the point of the switch to x86 I think, to make the powerbook into a pro offering that beats any consumer product desktop or laptop. Powerbook should definately have dual core, as will the powermac.