Nice pick up Guspaz! I didnt see the el5 and was scratching my head for a while. I have the exact same issue. Now sure if OP has solved this or not, but for those who stumble to this page (it was top in google search for me), you can change the repo for RHEL/CENTOS 5 to 6 in /etc/yum.repos.d
For me, I was running Centos 6.2 (cat /etc/redhat-release to find yours) and for some silly reason all my repos were for 5!!
Anyway, a quick bit of code to fix this up is below. It simply replaces el5 for el6 in all the files in that dir. Make sure you run it in /etc/yum.repos.d
for FILE in *; do mv $FILE $FILE.old; sed 's/el5/el6/g' $FILE.old > $FILE; done
Once done you can remove the *.old files if you dont need them as backups.