Mac OS X can easily convert a .dmg file to a .iso file. This can be useful to burn/mount .iso files on Windows, which were previously in .iso format. Just use the following command in a Terminal window:
hdiutil convert /path/to/read/dmgimage.dmg -format UDTO -o /path/to/save/dmgimage.iso
This will create a file named dmgimage.iso.cdr. To correct this, type:
mv /path/to/save/dmgimage.iso.cdr /path/to/save/dmgimage.iso