In the requires cinnamon requires networkmanager with and without a use flag. Thus even when -networkmanager is selected cinnamon still requires it. Removing networkmanager as a dep still lets it build correctly.
Fixed in cvs, thanks for noticing! > 04 Feb 2012; Alexandre Rostovtsev <tetromino@gentoo.org> > cinnamon-1.2-r1.ebuild: > Do not pull in networkmanager if USE=-networkmanager (bug #402153, thanks to > Lee Trager).