How to create a OS X Mountain Lion Bootable USB drive on Ubuntu?

As you want to create a LiveUSB you need a .iso image to make it. The Mac image is a .dmg file, so you must to convert this file into .iso, and then install it.

Install DMG2IMG

DMG2IMG ^{(1, 2)} will convert the .dmg image into a .iso image. To install open a terminal and enter this command: sudo apt-get install dmg2img

Convert the Image

With dmg2img you will create an .img file and this file will have the same format as an .iso file, you just need to replace the extension.

In the terminal enter the following commands or paste the script with the necessary modifications:

# Replace "OriginalFile.dmg" with the filename including the extension
# of your file. The extension will be replaced where necessary unless
# you want to specify the filenames yourself.

filename="OriginalFile.dmg"

# Replace with your file name and it will create the IMG file
dmg2img -i "${filename}" -o "${filename%.dmg}.img"

# Note that the extension of the first file is ".img"!
mv "${filename%.dmg}.img" "${filename%.dmg}.iso"

Make the LiveUSB with an ISO image

Now you already have the .iso image, with which you will make the LiveUSB. Several tools and answers exist to help you create bootable USB drives, some are specific to PC/MBR booting (Bootcamp) where Macs and newer PCs since Windows 8 require/support UEFI booting.

A related question: AskDifferent – Official way to obtain an OS X ISO file

why not just dd the .img file after conversion?

sudo dd if=/path/to/orig.img of=/dev/s**