There have already been three pre-release versions: the Developer Preview, the Consumer Preview and the Release Candidate. Seems to me there's already been ample opportunity to test things out and fix them where necessary.
By the way, I'm not running a release candidate but the actual final release of Windows 8. I have access to Microsoft TechNet and as such I'm running the RTM-version of Windows 8 - the version as it will be released to the public on October.
To rule out the problem was a temporary quirk, I'll try installing it again -- just to be sure
--- Edit: just tried it again. Apparently, something has changed in Windows 8, as you suggested. The installer seems to think it's running on a 32-bit Windows. I then tried to run the setup in Windows 7 compatibility mode, that worked as intended and it installed the 64-bit version.
Obviously, this is an issue that clearly needs to be adressed because it's occuring on the RTM-release of Windows 8. I'd be happy to volunteer to test a new installer.