I did it like this:

$ wget http://archive.ubuntu.com/ubuntu/dists/karmic/main/\
installer-i386/current/images/netboot/boot.img.gz
# zcat boot.img.gz > /dev/sdc
# eject /dev/sdc

On an old Thinkpad R40 the bios USB boot support was not that great - the bios wanted a mbr, thus I created the USB stick like this:

# cfdisk /dev/sdc
only one primary partition
file system type -> 06
bootable
write
# lilo -M /dev/sdc
# zcat boot.img.gz > /dev/sdc1
# eject /dev/sdc

Note, that this method uses the text-based network installer.

Of course, you have to adjust the USB device name (/dev/sdc above) and the Distribution name (Karmic is not 11.04).

I did it like this:

$ wget http://archive.ubuntu.com/ubuntu/dists/karmic/main/\
installer-i386/current/images/netboot/boot.img.gz

Or to choose current Ubuntu Natty and amd64 architecture:

$ wget http://archive.ubuntu.com/ubuntu/dists/natty/main/\
installer-amd64/current/images/netboot/boot.img.gz

# zcat boot.img.gz > /dev/sdc
# eject /dev/sdc

On an old Thinkpad R40 the bios USB boot support was not that great - the bios wanted a mbr, thus I created the USB stick like this:

# cfdisk /dev/sdc
only one primary partition
file system type -> 06
bootable
write
# lilo -M /dev/sdc
# zcat boot.img.gz > /dev/sdc1
# eject /dev/sdc

Note that this method uses the text-based network installer.

Of course, you have to adjust the USB device name (/dev/sdc above), the Distribution name (Karmic is not 11.04) and architecture (x86 vs. amd64).