I've noticed that pacman reports the architecture of a package given a pacman -Qi <pkgname> but how does it handle installing a 32-bit package on Arch64? I've never tried installing a 32-bit package and don't even know if it would work correctly. In theory, it would work but in compatibility mode. Would all libs required for the 32-bit app have to be installed as 32-bit versions also?
Interesting how pacman would handle this.
But Arch64 will never have multilib support. So you won't be able to run 32bit apps if they need certain 32bit libs. If you want that go with one of the bloated multilib "intermediate" 64bit distros.
I don't feel that 32-bit lib support is that important as most apps should be happy to recompile linked aginst the 64-bit versions of all libs. The only potential conflict I could see is proprietary apps, but those often come shipped with their own versions of libs anyway.
If pacman would allow you to install a package for a different architecture normally, perhaps it should warn users that it's installing a non-x86_64 package. Unlike some pacmage managers pacman doesn't use the architecture as part of the filename... ie no .i686.pkg.tar.gz or .x86_64.pkg.tar.gz. This would be difficult to change but perhaps a warning would be a simple solution to a possibly complex problem.