If you use a non-Linux desktop (we call this the host OS), you can find useful information in this article about connecting to your Openmoko smartphone or running/developing Openmoko software.
Mac OS X
Covered elsewhere; see MacOS_X. They have dfu-util and ether-gadget working.
The umodem(4) driver works with the u-boot console. When the umodem driver claims device 0x1457:0x5118, it becomes inaccessible to libusb-based applications such as dfu-util. The cdce(4) driver works "out of the box" with Openmoko's ethergadget interface, so you can ssh in, run remote X clients, scp in or out, etc.
Ian Darwin has a port of dfu-util partly working (list works but not download).