quick search shows that this has been requested before, but not for a while. Please consider adding Quantum GIS to the stable - QGIS is open source (GNU Public License) and runs great off a thumbdrive. Well it used to at any rate. Recent versions are not so happy (and probably older versions were never properly portable).
Release notes for the current version (1.6) explain how it can be made portable (see http://www.qgis.org/en/about-qgis/qgis-releases.html):
"•Added --configpath option that overrides the default path (~/.qgis) for user configuration and forces QSettings to use this directory, too. This allows users to e.g. carry QGIS installation on a flash drive together with all plugins and settings."
I'm sure this means something to someone!
For more info please see: http://www.qgis.org