Unable to create an Ubuntu 12.10 USB stick on OS X

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http://askubuntu.com – I'm following the instructions to create a bootable Ubuntu 12.10 USB stick on OS X found here. I can do step 3 and hdiutil appears to work fine, but when I go to mount the resulting ubuntu.img file I get the same "no mountable file systems" error that I get when I try and open the ubuntu.iso file. No correctly sized volumes show up in diskutil list either. Any ideas? Using the most recent ubuntu-12.10-desktop-amd64.iso on Mountain Lion 10.8.2. (HowTos)