If you have Lion, you have Apple hardware. In which case you're better off posting this in the Mac forum.
My previous understanding is that UEFI booting was only supported with x86_64 media, not i386 media. So that could be the issue.
Be advised that for Apple hardware, only EFI boot supports USB sticks. And EFI booting Apple hardware with OS's other than Mac OS, in my experience, is highly variable whether you will achieve success, depending on the mode. I have two laptops and so far can't get them to fully successfully startup to a GUI when EFI booting.
And Apple's CSM-BIOS boot (for Windows support) doesn't support booting from a USB stick, but both of my Macbook Pros can boot Fedora find in CSM-BIOS mode boot.
---------- Post added at 05:07 PM ---------- Previous post was at 02:49 PM ----------
OK so I've looked at this again, and while in theory it applies to the DVD install media as well, I've only tested it on LiveCD.
First within Mac OS X you can use this command to determine if you have 32-bit or 64-bit EFI.
ioreg -l -p IODeviceTree | grep firmware-abi
It should come back with either EFI32 or EFI64.
If EFI64 you need to use x86_64 install media. Otherwise, you're going to have to hack together an x86_64 EFI bootloader, with i386 install media if you really want an i386 installation for some reason.
If EFI32 then you can should be able to use i386 install media, but as I don't have such Apple hardware I haven't tested this.
To produce the install media you should use livecd-tools 17.7-1.fc17 currently in updates-testing, which has bug fixes for creating EFI bootable media. I used this command and it does work. You may use an iso image instead of /dev/sr0, and sdb in favor of your actual USB device.
livecd-iso-to-disk --format --efi --overlay-size-mb 500 /dev/sr0 /dev/sdb
use livecd-iso-to-disk --help for more options.