If you get that error when attempting to install a deb file built using Qt Creator (Maemo project) on Nokia Qt SDK it can be because the debian control file has "Section: unknown" instead of something like "Section: user/development" or "Section: user/office" etc.

Though the debian file has a section category it needs to be something other than unknown to install without having to gainroot and install via xterminal.

More info here: http://developer.qt.nokia.com/forums/viewthread/1972/