Ok, there's been discussion, including, I think, on the wiki web page, that syslinux is not correct. At any rate, after enough experimentation, I have a working install on a USB key. The procedure is:
Using fdisk, partition your key: one partition, VFAT (type b, and toggle the bootable flag), about 9M or 10M, and a second partition big enough to hold a DVD. That should be type 83 (Linux native).
mkfs -t vfat <the DOS partition) mkfs <the Linux partition
liveCD-iso-to-disk <.iso, w/ full path> /dev/<USB DOS partition> mount <USB Linux partition> /mnt cp <CentOS.iso> /mnt/
You're done. You do have to tell it "install from hard disk", and then point it to the Linux partition.
I just did this and tested it (or at least as far as the "searching for existing CentOS installations).
mark