In perhaps what should be taken in the strictest definition of a rumor, one Xlr8yourmac.com post indicates that one Apple engineer suggested that the PowerMac G5's may be difficult to upgrade (Processors) due to cooling/venting requirements:
Obviously, more information will become available in the months to come, as machines become available and upgrade vendors gain access to these machines.
One thing he said is that it will be extremely difficult to upgrade the chips in this machine. Each computer is uniquely paired with its processors to have perfect venting. Changing out processors would throw off the chip that handles the G5 cooling, resulting in (at best) the computer running for a while then shutting off, or (at worst) a non-booting machine.
Obviously, more information will become available in the months to come, as machines become available and upgrade vendors gain access to these machines.