GPSy (TM) -- Hardware FAQ: "How do I hook up a GPS receiver to my USB port?
Apple's new iMacs, G3/G4 desktops, and PowerBooks/iBooks come with USB ports rather than the standard serial ports found on older Macintosh computers. This presents problems as GPS units expect standard serial ports for communication. While GPSy and GPSy Pro themselves work fine on these new Macs, you'll can't directly hook your GPS unit up to your USB-only Mac as there are no GPS receivers with USB ports right now."