I was pulling my hair out trying to figure out why a brand new fresh install of CentOS 5.2 would stay stuck at the grub menu waiting for me to press enter. I tried everything I could think of.

Turns out, if you have a PS/2 mouse and keyboard plugged in, the autoboot does not work. With no mouse and keyboard, it works fine. Kinda a silly feature if you ask me, but posting here for my own reference and others' reference.