If you can persuade a friendly service centre to order and sell you the receiver from an iMac G5 and its cable, I can tell you the pinouts to hook it to USB.
Otherwise, Front Row is built into Leopard. If it detects a compatible receiver, it works flawlessly. Consequently, 3rd party receivers for older Macs and Mac Pros will be available soon after Leopard launches. Maybe Keyspan will write some new drivers.