Function names with a period character aren't being recognized and highlighted by vim, because of a bug in sh.vim which is inherited by ebuild.vim. For an example, see games.eclass:92-93 in the portage tree - dogameslib.a() is unhighlighted.
In SVN r86:87. I'm just overriding this just for ebuilds for now, I'll have a go at getting it pushed upstream when the next batch of runtime changes goes through.
Release 20041127, thanks