I'm of the "adamantine bypasses DR" camp.
Touch attacks ignore armor. That happens to include "natural" armor.
An enhancement bonus is needed to bypass DR. Adamantine has a "natural" enhancement bonus. It's still an enhancement bonus, just like natural armor is still an armor bonus, so that works too.
Now here's something that just struck me as odd...
When you put a magical enhancement bonus on an adamantine weapon, it "overlaps" with the adamantine bonus, hence the bonuses must be similar (same bonuses don't stack). Another argument for allowing adamantine to bypass DR.
If adamantine is truly a different bonus from magical enhancement, then perhaps it should stack. An adamantine weapon with a +2 magical enhancement could provide +4/+4 to attack/damage, but only be a +2 weapon for purposes of DR.
Anyway, just some random thoughts...