The Fedora 12 iso files are hybrid files. You can burn them to CD or put them on USB stick. Creating a bootable Fedora 12 USB stick is as simple as:
dd of=/dev/sde if=~/iso/Fedora-12-x86_64-netinst.iso
There are no fancy commands to prepare the USB or make it bootable. Just good ole dd. It's worth noting that the DVD isos do not have this feature, only the CD images.