diff against a tag ?

Maybe I missed this in the discussion on tags recently
(I kinda tuned out after a while on that thread).

Is there an easy way to express 'show me the diff
between HEAD and 2.6.12rc3' today ?
Looking at the commit for rc3, theres nothing obvious
to me distinguishing it from any other commit
other than the "Linux v2.6.12-rc3" in the description,
which makes it somewhat difficult to automate.

		Dave