There are two areas in which other OSes can take a part in OpenMoko. First, not everybody will use Linux to download (and/or develop) software into their phones; we call this the Host OS. Secondly, it is possible that some people may wish to run alternate OSes on their phone itself.
Alternate Host OS
(link to this page...)
Mac OS X
Covered elsewhere; see MacOS_X. They have dfu-util and ether-gadget working.
The cdce(4) driver works fine with OpenMoko's ethergadget interface, so you can scp ipkg files then ssh in and install them.
Ian Darwin has a port of dfu-util partly working (list works but not download).
Alternate Software In the Neo Phone
At JavaOne, Sun demonstrated JavaFX running in the phone. Although based on a similar Linux kernel, this is a completely different user-mode stack.
This Linux-based project claims to have "experimental" support, but it is based on 2007.02 firmware.
Windows CE runs on some Arm-based handhelds, so it is theoretically possible.