So, the yum package manager (and, I assume, RPM underlying that) has a fairly neat (to my naive eyes, anyway) implementation of multiarch support -- on an amd64 system you can install i386 packages, and they go into a separate lib tree, and there's no conflicts or unhappiness. It even installs both the i386 and x86_64 versions of libraries automatically when you ask for them, which is a nice touch.
However, the dependency resolver doesn't quite seem to have gotten the hang of some of the corner cases. For instance, if you want to install a noarch package that depends on an architecture-specific package, but for some reason only the i386 version is available (say, your local package repo hasn't quite built the x86_64 version yet) then yum will happily install the i386 version and say "OK, dependency resolved". Nice try, but no cigar. What makes it a bit more irritating is that once you work this out and the x86_64 package is available, you need to remove the i386 version of the package by hand in order for it to pick up the x86_64 package and install it. Just asking yum "hey, can you install this package" results in "nope, it's already installed, go away".