The Apple Pro Speaker delivered with the Apple Cube had an USB, with an external amplifier. But as far as I know, you couldn't buy it seperately.
Those that came with the flatpanel iMac G4 are also sold seperately, but they have a special mini-jack. They are passive, means without amplifier. The only Macs that have the built-in amplifier (and therefor have that special mini-jack connector) are the iMac G4 and the PowerMac G4 (from the Digital Audio models to the MDDs).
The iMac G5 doesn't have that connector (and no integrated amplifier), so the Pro speaker won't work with it.