trac 0.10.4-21 depends on /usr/lib/python2.5/site-packages/neo_cgi.so, which is provided by python.i386 but not by python.x86_64. If architecture-dependent code has been added to trac, it mustn't remain a noarch package anymore but provide packages per architecture instead.
neo_cgi.so is not provided by Python, it's provided by the ClearSilver package which is available for x86_64. I think that your logic regarding arch/noarch packages is incorrect. If you were right, it would be impossible to have any noarch packages at all. I think that the problem is that you have the i386 version of python-clearsilver installed but not the x86_64 version. Try removing any clearsilver packages and re-installing them. Unless something is wrong with your yum configs yum should install the x86_64 versions.
Sorry Jeffrey, my fault - to my surprise, I just figured out atrpms provides the faulty trac with arch-dependent python-clearsilver bindings, since I didn't suspect a 3rd party repo would provide trac (I wonder why??) I filed the bug here instead of investigating the source. Thus I'm closing the bug. Thank you for your good work and maintaining the trac package!