NOTE: I am switching to Emacs so won't be maintaining this anymore. Anyone else should feel free to take it over.
Mickaël Delahaye has a plugin for Gedit 2 to force off spaces-instead-of-tabs for Makefiles. This is a similar plugin for Gedit 3, rewritten from scratch since all the APIs have changed.
- Version control: git://git.gitano.org.uk/personal/liw/makefiletab3