Need to fix power first
The current prescott design draws a fair amount of power. When 64b is enabled, I suspect the power will go up a bit. I think Intel needs to reduce power consumption before they can think about implementing features that will raise it.
Xeon has always been specified with a higher power budget than Pentium, so I suspect they can hit the Xeon targets with 64b enabled, but this does not guarantee they are ready for the desktop... yet...
--Alan