I like the idea of having a yum repo file pushed out by redhat, although I'm not sure if they'd go for it or not. If not, I think a good idea may be for us to release a package like legacy-yumconf-fc3 which would provide the file /etc/yum.repos.d/legacy.repo. CentOS does something similar if you want an example. If we get real ambitious, we could even stick our GPG key on the system (they'd still have to manually import it to yum/rpm).
I was going to suggest the same, as I'd just done this to include the Livna repo on my FC4 system:
<http://rpm.livna.org/configuration.html>The FC4 RPM includes both the repo file and the GPG key, and the first use of yum to install a package prompts to install the key into RPM.