In this post, I used Fedora 11 (x86 DVD) for example.
Start UNetbootin then select the ISO diskimage and USB Driver, if you're sure of your choice, click "OK" and wait until completed.
Now, It's completed and you can do what you want ! :D
Remember to set your BIOS/CMOS 1st boot on USB Driver or nothing will happen !! :D
Read more and download UNetbootin at http://unetbootin.sourceforge.net/