In order to create an Intel Mac bootable USB stick, it needs to be setup for EFI/GPT. This should work as of Fedora 10, but it's not automatic; you need to create a USB stick specifically configured for this purpose.
According to the Fedora 10 release notes, you can do this using the
livecd-iso-to-disk tool like so:
livecd-iso-to-disk --mactel /path/to/live.iso /my/partition
replacing the path and partition as appropriate. However, this post suggests that the MBR may also need to be cleared (destroying all existing data on the USB disk):
livecd-iso-to-disk --mactel --reset-mbr /path/to/live.iso /my/partition
Other details on creating a USB stick are in the Fedora wiki, though curiously nothing about support for Intel Macs. I've had success with this method in the past, though it seems somewhat hit and miss. As far as I know, it's not possible to create a USB stick that will boot on both Intel Macs and non-EFI machines.