The Sunday night session of the QGIS hackfest resulted in a new release of the Mapfile Tools plugin.
This QGIS plugin allows you to display an UMN Mapserver mapfile in QGIS without running a Mapserver instance. It depends only on Mapscript (apt-get install python-mapscript on Debian/Ubuntu) and allows you to zoom and pan on the mapfile layer.
In release 0.6, an output window has been added, which shows error messages and detailed layer information. This makes it a convenient tool to test your mapfiles.