Booting off USB drive

[Samuel Donham]

Has anyone tried booting a whole distro off an external USB hard drive?

I have Fedora 3 installed on my laptop's (IBM t42) internal hard drive.
I need to also boot into Redhat ES 3 so I can develop on that as well.
Unless someone knows of a better way to access both distros, I thought I
would just install ES 3 on an external USB hard drive and boot from that
when I need to.  

Thoughts?  Advice?  Has anyone tried something like this before?  I
noticed some references on the net, but they are spotty at best.
Thanks!
-sam