As I recall, markB is using USB 1. A fairly lengthy search has not indicated conclusively whether or not USB 1 works with the Uno.
I think USB 2 devices are supposed to be backwards compatible with USB 1 hosts, but perhaps I am wrong there.
Is there any user here who can conclusively say "I have USB 1 and the Uno works" or alternatively "The Uno does not support USB 1 ... it is documented "? I can't offhand see from the ATmega8U2 specs confirmation one way or the other.
It might be a firmware issue. For example, I recently discovered that the HardwareSerial library does not work correctly when programmed at 300 baud (you get 777 baud instead). Now most people probably don't use 300 baud these days so this might have gone unnoticed (well, it has gone virtually unnoticed, right?).
Similarly with USB 1, I expect that hosts that can run the Arduino software, but only have USB 1 ports, are rather thin on the ground, and thus it might not have been tested.