It seems that some people are having difficulty making their older Macs mount their new fourth-generation iPods via a FireWire connection. The solution to the problem lies in the iPod's box.
Specifically, these iPods don't appear in iTunes, on the desktop, or in Apple System Profiler when connected to a Blue and White Power Mac G3 or a Power Mac G4 (PCI Graphics). To make your iPod's presence known, connect the iPod to your Mac with the included USB cable. Run the latest iPod Updater (currently 2004-08-06), and update the iPod. According to Apple, after such an update the iPod will be recognized by the computer when connected via FireWire.