Mac ignores all startup key combinations!

I don't know exactly when I started having this problem, but I remember when I used Tiger I could do it fine. I can't startup in single-user mode (hold down CMD S) or the startup disk select one (hold down OPTION). My iMac completely ignores those commands and it goes straight to the desktop. I want to startup in single-user mode so I can run Applejack but my Mac is completely ignoring the CMD-S command while booting.

This works perfectly fine on my Mac Mini also running the latest version of leopard, but something is wrong with my intel iMac and I don't know what.