I never do RPM kernel upgrades. I always download new kernel sources from kernel.org, compile them, and install them. If you want to make it so that it seems that the kernel never existed, I would:
1) Delete the compressed kernel image in /boot (vmlinuz-2.6.6-1.435.2.3).
2) Delete the entry in /boot/grub/grub.conf.
You can also delete the sources, if you don't plan using them ever again. If you want to actually recompile it all, do "make clean" in the source's directory, and then "make && make modules_install" to compile again. To my knowledge, that's how you would "uninstall" a kernel.