I did a hike in the Sierra Nevada last year with my hooded MB UL Down Inner and, like Ben and others, found that I was good above 40° at rest and even a bit below 30° while moving. Anything else and I needed more layers. I, too, almost never wear just the UL Down Inner without a raincoat, unless I'm quite still and in camp or in completely exposed areas (to prevent snags on branches, etc.).
For further reference, going over Mono Pass (12.000') in sleet, 30° temps, and 30+mph winds, I had 2 baselayers, the down jacket, an eVent rain jacket, both hoods up, a Buff covering my face, gloves, pants, and GoLite Reed rain pants...and while hiking, I was pretty much balanced between cool and warm. But that was going downhill. ;-) It was a layering strategy that worked well for the trip, which included morning temps in the 20's (not toasty, but adequate with hot food). I'd have been warmer and happier in the cold mornings had I used by WM Flight jacket! :-)