I brought up the issue of arch-specific Requires some months ago in response to problems I experienced with xulrunner. This issue has reared its head again with gvfs on F13:
https://bugzilla.redhat.com/show_bug.cgi?id=573183
I've put up this draft:
https://fedoraproject.org/wiki/PackagingDrafts/ArchSpecificRequires
Comments welcome.