You can use the OSX Bootcamp Assistant to accomplish this.
Copied from:
http://kb.iu.edu/data/bciz.html
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Creating a Windows flash drive installer on an OS X computer
Note: In Mac OS X, the option to create a Windows 7 flash drive will only appear if the computer you're using does not have an optical drive. On Macs with optical drives, the option will be either missing or grayed out.
In Mac OS X, to create a bootable Windows 7 flash drive:
From the Finder, open the Applications folder, and then Utilities.
Open the Boot Camp Assistant, and then click Continue. The next screen should give you a list of options.
UITS recommends making the USB drive installer first. You can skip the actual Windows install and downloading the support software at this time, as it will be easier to do both later. To proceed:
Verify that the USB drive you will be writing to is plugged in.
Uncheck the Install Windows 7 and Download the latest Windows support software from Apple options.
Check Create a Windows 7 install disk and click Continue.
Your USB drive should be listed in the "Destination disk" area. Use the choose button to browse to your .iso file; after selecting it, click Continue.
If prompted, confirm your action and/or provide an administrator's password. The process of writing the .iso file to the USB drive can take twenty minutes or longer.