Jack Norris of Vegan Outreach has just completed the first draft
of an extensive review of the scientific literature on vitamin B-12.
You can read it (fair warning: it's *very* long) at:

http://www.veganoutreach.org/health/b12.html

The paper is a first draft, with some typos. But it is
very comprehensive and worth the effort to review (although
you might want to cover it in sections and not in one go).

Jack has done an excellent job on this paper.

Tom Billings