Try another keyboard? And re-test those cables on other machines.
I have the newest 27" iMac. I love the machine but I am having a problem getting the keyboard connected using a 6 foot USB extension cable. This cable worked with my 24" iMac just fine, but the keyboard will not connect to my computer with the extension cable. The keyboard works without it, but not with it. I have tried one of the other USB ports on the iMac with no joy. I bought a new extension cable and that one does not work either. Ideas?