Just chiming in here.. do you happen to have any HP Printer drivers installed on these Macs? I ask because some other USB devices have lost their ability to be seen after OSX 10.4, in cases where certain HP printer drivers have been installed.

e.g., http://discussions.apple.com/thread....readID=1250686

In some initial googling, I've been able to find other USB devices that have suddenly stopped being recognized by the most recent versions of OSX. Perhaps as we continue to search we will turn up some common solutions. The link above is just a single example of a solution--I expect there are more and that most would not be immediately intuitive. Please let us know if you also are able to find any kind of solutions via other USB hardware problems on other forums.

Also, if you haven't already, you could try updating or re-installing your computer's USB interface drivers.