I tried several methods last night including Unetbootin, and this is the one that worked best.
Basically copy the ISO on the USB root directory, add vmlinuz and initrd.gz from main/installer-i386/current/images/hd-media/. Then add syslinux.cfg with:
default vmlinuz append initrd=initrd.gz
If you have boot problems you might also need to run install-mbr /dev/sda from the mbr package.