This is a very interesting area and relates to a discussion I've been following on the craftbrewing discussion list. Decoction mashing involves removing some of the mashed grains in a small amount of liquid, boiling then returning to the tun. If boiling in its own right brings out tannins then decocted brews should be harsher than single infusion - but I've read that this is not the case. Rather decocted brews may well be smoother than single infusion mash brews.
One explanation I read relates to mash ph. I'm a bit vague on this but from what I remember - so long as mash ph is below 5.7 or so, tannins won't be extracted. So when mashed grains are removed from the tun for decoction - because only a small amount of liquor is removed at the same time the ph lowering effects of the grain are in a sense magnified (as there's a much greater grain to liquor ratio).
ie if it's PH level that determines tannin extraction rather than boiling, then boiling is fine so long as ph is ok.
It's an interesting argument since I have often read never to boil grains. I think I've correctly represented the above argument but am happy to be corrected.