OK, I've been looking at this more closely, inspired by these
instructions for getting iTunes working in XP x64.
http://yukichigai.googlepages.com/iphonex64
The Boot Camp that comes with the Mac Pro is apparently
Boot Camp 2.0.1, and that includes support for Vista x64.
Check out this MacRumors thread to see how to get
your own copy of Boot Camp 2.0.1.
https://forums.macrumors.com/threads/418220/
(I wonder how Apple plan to distribute an update to Boot
Camp 2.0.1 officially? I have Leopard and Boot Camp 2.0.0,
but haven't created a Boot Camp partition up until now because
all I have is XP x64 and Vista x64.)
Using the same Orca MSI editing software used to get
iTunes to install, I looked around inside the BootCamp64.msi
file that comes with Boot Camp 2.0.1.
Looking for references to VERSIONNT (which has a value
of 502 for XP x64), it looks like Boot Camp x64 is a bit more
complicated than iTunes x64, with more driver installations
controlled by the version, but it looks do-able.
Maybe someone with more free time than me and a Mac
to experiment on can investigate this more thoroughly
and give it a try.
(By the way, I think one reason why Apple is hestitating
in generally releasing Boot Camp x64 is the fact that the
Vista SP1 release is imminent. A lot of the version tests
in the MSI test service pack level as well as version
number. Moreover, service pack levels in XP x64 do
not correspond directly to those for regular XP, such
that XP SP2 is (about) equivalent to XP x64 SP1.
If you look at the version testing syntax in the MSI file,
you'll see how it can get very complicated very fast with
all those versions available.)