[quote]
I wouldn't be suprised if there is something that prevents updates going too far.
[/quote]
You have exclude=kernel* in several stanzas in the files you quoted above. You won't receive any kernel updates while those are there. No idea if there is a specific reason why they should be present but the worst thing that can happen is that you remove them, update and reboot and something fails or stops working at which point you reboot again and choose your current kernel from the list and boot up on that.
installonly_limit=5 tells yum to keep the latest 5 copies of kernel-x.y.z installed. If you install a 6th then it will remove the oldest one to make room. The kernel packages are listed automatically in 'installonlypkgs=' list so they are not updated but installed in parallel if a new version comes along - precisely so that you can keep multiple copies of the kernel installed and fall back to an older one if the latest fails to boot.
I think it might be interesting to see the output from `rpm -qi kernel` to see if the current kernel you are running is built by CentOS or by trixbox. If it's built by CentOS (i.e. Build Host says *.centos.org) then the new one stands a better chance of just working than if trixbox have taken the source RPM and rebuilt it with unspecified changes in it.