Well I just ordered one of these back pocketed merino T shirts (which they call a jersey) from Rivendell.

http://www.rivbike.com/product-p/at13.htm

It's supposed to be very fine & thin so I'm hoping to use it quite a bit on moderate summer days. I'll cold wash it and hang dry it and hope to heck it doesn't shrink up my back since they don't offer a tall version.