"The Icebreaker does get better with wear. I washed mine in Nikwax Wool Wash on gentle cycle, and it felt much softer afterward. Your skin also gets used to it pretty quickly as well."
Okay, this story of skin adapting is a bit more harsh than simply wearing Merino...
While getting my BS Chem Eng degree, I did a 9-month co-op for, of all things, a fiberglass insulation plant. Talk about itchy skin... get too close to the machine and you pick up itchy fibers just from 'overspray' in the air. Anyhow, after a month of working there, I could shove my arm elbow-deep into a mound of poor quality insulation (poor quality is much more ithcy than good quality) and not be ithcy at all..
Anyhow, you skin should adpat as long as it's the fibers than are causing problems and not the lanolin (but that should be a different feel, more rashy than itchy). I would suggest wearing the shirt regularly (say every friday as an undershirt) to get and stay used to it. If it still itches after a month or so, you may be too sensitive, but I bet you'll adapt.