I copied exactly where Axel provided into atrmps.repo. I have the line:
atrpms.repo:baseurl=http://dl.atrpms.net/el5-x86_64/atrpms/stable
but the rpms that are being flagged as updates pretty much all have fc5 in their names.
e.g.:
---> Package mplayer-fonts.noarch 4:1.0-7.at set to be updated ---> Package mplayer.i386 4:1.0-60_r23482.fc5 set to be updated ---> Package zonecheck.noarch 0:2.0.4-3.fc5.at set to be updated
???