They are not dumb, but it does happen, a few months ago when BETA 2 was
released, a lot of persons didn't even realize you had to boot off the disk
to install Vista x64. So, for you to assume that everybody just gets it is a
bit naive. I think the Company would consider it a complete waste of disks
to include both when you will be able to install only one, because you only
get one license. The packaging will give the buyer the information they need
to decide which platform they believe is right for their computer, in fact,
a large portion of PC's out there are 32-bit which makes sense to sell just
1 disk with 1 license.
Also, another reason I believe they won't include both x86 and x64 disk in
the packaging,components such as Anytime Upgrade might confuse users into
thinking you can upgrade from Vista x86 to Vista x64, which is not possible.