On my D50 (not that different from the D80) there are two different modes for USB operation -- Mass Storage, in which it functions as an SD reader, and PTP, in which it functions as a device from which images can be retrieved using Image Capture. In PTP mode, if I recall correctly, you can still shoot while the camera is plugged in.
But it sounds like you might benefit from Nikon Camera Control Pro, a program ($57.99 at MacMall) that lets you upload photos to your computer as they are taken. I believe there is a 30-day free trial available from Nikon if you want to give it a try; I haven't ever really messed with that sort of stuff, since I generally don't have my computer with me when shooting.