This article is from the Vegetarianism FAQ, by firstname.lastname@example.org (Michael Traub) with numerous contributions by others.
Nutritional yeast (saccharomyces cerevisiae) is a food yeast,
grown on a molasses solution, and comes in powder or flake form.
It has a pleasant-tasting, cheesy flavour and can be used directly
on vegetables, baked potatoes, popcorn and other foods as a
condiment. It is different from brewer's yeast or torula yeast.
It can often be used by those sensitive to other yeasts.
Ms. Carlyee Hammer at Universal Products (the parent company of
Red Star, (414)-935-3910) indicates that ONLY ONE variety of Red
Star nutritional yeast (product number T-6635+) is fortified with
B12 at the level of 8 ug/g.
Ms. Carlyee also claimed that other varieties of "nutritional"
yeast contain vitamin B12 at less than 1 ug/g, but was unaware
whether this was determined by microbial assay or not. Microbial
assays for vitamin B12 are no longer considered reliable due to
problems with the cross-reactivity of corrinoids. She indicated
that Hazelton Laboratories (608-241-7210) did the assay.
From the above two paragraphs, one might conclude that Red Star
T-6635+ nutritional yeast, and probably no other variety, is a
reliable dietary source of B12 at this time.