Just so I don't have to struggle to remember this any more:
- grab a disk image of the desired net installer (e.g. http://archive.ubuntu.com/ubuntu/dists/lucid/main/installer-i386/current/images/netboot/boot.img.gz)
- insert usb flash drive, find it using diskutil list, then unmounting it (e.g. diskutil eject /dev/disk3
- write the contents of the uncompressed file to the raw flash drive -- not a partition/slice (e.g. dd if=boot.img of=/dev/disk3)
Boot your x86 box from the USB stick.