Typically, when you run the Bootcamp assistant on Macs that come as standard with an optical drive, you won't be given an option to create a USB Windows install disk. There's a few reason you might want this, for example if you've replaced your optical drive with a second hard drive. This guide will show how to force this option on Mountain Lion. You'll need Xcode (free from App store), although you can also make the modifications using nano via the terminal if you're an advanced user. I'll only be going through how to do this with Xcode.
Step 1: Permissions
Go to Applications > Utilities > Boot Camp. Alt-click and select "Show Package Contents". Alt-click on the folder 'Contents' and select 'Get Info', and then add 'Administrators' to the 'Sharing & Permissions' with read & write privileges. You will have to click the padlock and enter your administrator password to do this. Do the same for the file named 'Info.plist' that the 'Contents' folder contains.
Step 2: Find your system version
Click > About This Mac > More Info > System Report and then copy the first section of the 'Boot ROM Version' string.
Step 3: Edit 'Info.plist'
Open 'Info.plist' with Xcode. Expand "USBBootSupportedModels", add a new item by pressing the '+' symbol. You will be prompted to unlock the file, click unlock. If you get a permissions error, check that you've completed the previous step. Paste the first section of your 'Boot ROM Version', for example 'MBP81', and then save the file.
Startup Boot Camp Assistant, and you will now have the option to create a Windows 7 Install Disk. To turn a Windows Install DVD into an ISO image, you can follow this tutorial: How To Create Windows-Compatible ISO Disc Images on Mac OS X
I've replaced my optical drive with a second harddrive, so this was necessary to do for me to install Windows, as Microsoft doesn't allow you to install it via an external optical drive regardless of system platform. The Apple external superdrive will work with MBAs and rMBPs, but only because Apple have done something special with the drivers.
Edited by Akamatsu, 21 October 2012 - 04:01 PM.