This does the same thing as:
hdiutil convert /path/to/filename.dmg -format UDTO -o /path/to/savefile
but neither that nor this makes an ISO9660 *filesystem*... they both are still Mac OS Extended (HFS+). I can use neither resulting image on WinXP!! Crap... to the person who asked about .dmg, you can't just rename those b/c they are compressed. If you make a .cdr ("CD/DVD master") and rename it .iso, that will burn in Windoze programs, but the damn volume is still hfs+ and therefore not readable when you put the disc in a PC optical drive. I typed "man hdiutil" and there is a "filesystem -iso" but I can't figure out where to put it in the command b/c I think Apple changed the hdiutil syntax. :(