I found this post from darkness very helpful when I moved from an older RHEL to CentOS 5.4 on a Dell PowerEdge 2650 server:

http://darkness.codefu.org/wordpress/2006/09/22/247

Adding: i8042.dumbkbd=1 to the kernel command line (it’s in /boot/grub/grub.conf) worked exactly as advertised. Apart from the keyboard input being a bit slow, and not being able to press the TAB key to auto-complete my commands, it’s great.