This entry is part 17 of 19 in the Bash-whacking series
sudo apt-get install dmg2img
dmg2img /path/to/image.dmg
sudo modprobe hfsplus
sudo mount -t hfsplus -o loop image.img /mnt
 

The .dmg archive is now mounted at /mnt. You can browse it either via command-line or via Nautilus.

Courtesy of http://iremedy.net/blog/2010/11/how-to-mount-a-dmg-file-in-ubuntu-linux/