I don't think many people would be interested but still leaving a note. I got Debian/Lenny to run on a USB flash disk (1GB) with XFCE4 on my MacBook Pro. I know this has been done earlier and there is lots of documentation for it. But my only requirement was that the installation should not involve any changes on the MBP internal hard disk whatsoever.

I am documenting my procedure for the installation here. It is just a quick write and should be considered a first draft. You can leave suggestions to improve the document below.

PS: Steve, the CSS restyle went on hold in the last few weeks due to busy time in family. Will revive it in a couple of days.