I noticed this when I started swim training with a club for the first time in my life. If I hadn't eaten enough carbs before the session (which was 1 - 2 hours long and quite tough) I would get this smell and taste in my mouth.
I looked up the explanation and it made sense to me (I am a Biochemistry graduate so it ought to!)
I find that eating cake before a swimming training session stops this happening, and also over time as I became fitter it happened less and less (without the cake requirement).
(I've never had this effect from running - don't think I run far or hard enough to deplete my carb stores!)
ETA I see the RW article recommends an apple, not cake. Ah well...
Edited: 29/10/2010 at 16:24