I always wear the wool layer next to skin - in winter, and now in summer too! We've been enjoying temps in the mid-80's and 90's, and wool (the lighter weights - 150 for Icebreaker gear, or the equivalent in Ibex) is very comfortable. I even won over my skeptical husband -I got him some shirts in the latest sale, and theyve been in heavy rotation this week.
In hot weather I'll wear a merino T-shirt or tank alone, over my usual undergarments (sport bra for workouts, regular one if not). Occasionally I'll throw on a button down shirt for sun coverage, but since I don't yet own a merino button down, that ends up getting sticky.
For cold weather, I layer merino like crazy: tee or tank, then L/S hoody, and if need be a mid weight zip-up jacket. But yes, the key for me is to give the wool a chance to wick away the moisture, which only happens next to skin.