Steve said the Intel transition will be completed by the end of this year. With yesterday's release of the new macbooks, the towers are the only systems left to change over.
My guess is that Steve is waiting for production ramp-up, so they will be available in mass qtys upon release, that and havng enough of the newest, fastest chips intel has available, since the towers are likely to represent a complete re-design from the ground up, which takes time, lots of it.......
I suspect that Steve does NOT want to disappoint professional users by coming out with some lame, retro-fitted G5 case where the only significant change was the cpu......thats not his style 😛