Well, it turned out that ipmi_si.ko was not broken after all in the new CentOS 6.3 kernels! Red Hat changed the module to be included in the base kernel and not as a module, and therefore the way in which I was specifying parameters for it (through modprobe.conf) no longer worked.
Instead what you do to make it work is to append the following to the kernel options in /boot/grub/grub.conf ;
ipmi_si.type=kcs ipmi_si.ports=0xca2
And then all is good!