My Mac Mini arrived today!
It's tiny, silent and most of all, works with the Belkin Omnicube KVM! It's actually a bit more complex than that: The keyboard is an old AT keyboard (with the old style pre-PS/2 DIN interface) which is converted to PS/2 for the KVM. The mouse is a Microsoft Optical USB mouse which is also converted to PS/2 for the KVM switch. The Omnicube is a four port PS/2 switch which is then plugged into a two port PS/2 to USB converter (that I found in Maplins) and then plugged into the Mini.
The mouse is fine. The keyboard is almost completely sorted. For a start, it finds the UK pound sign correctly, but has problems with the positioning of the speech marks (should be above 2 on the UK keyboard, but is found in the US position), the @ sign (which is now above the 2) and the page up/down, home and end don't seem to work.
Apart from that, it's great. Now I just need to learn Mac OS X