Gentoo Linux LiveUSB HOWTO

"This document explains how to create an unofficial Gentoo Linux LiveUSB or, in other words, how to emulate a Gentoo Linux LiveCD using an USB flash disk. This is particularly useful for installing Gentoo Linux on a computer with no CD-ROM drive.

While the instructions found in this document aims at emulating a Gentoo LiveCD using an USB flash drive, they should work for any arbitrary block device as long as you adjust the device names accordingly."

http://www.gentoo.org/doc/en/liveusb.xml