There are actually three boards (that I know of), two from Gigabyte,
http://www.gigabyte.com/products/product-page.aspx?pid=4755#sp
http://www.gigabyte.com/products/product-page.aspx?pid=4754#ov
the other is from ECS,
http://www.ecs.com.tw/ECSWebSite/Product/Product_Detail.aspx?DetailID=1508&CategoryID=1&MenuID=172&LanID=0
so far I haven't seen one from Z87 chipset, and there probably wouldn't be. Since Z87 are mostly used for K designated processor which requires a lot of power, so it wouldn't be a good fit for thin mini-itx use case.
Too bad there aren't any reviews for the motherboards listed above yet. I really hope someone review or blog a build for at least one of them, since I'm planning to buy a thin mini-itx motherboard 3-4 months from now.
Oh, and in case anyone didn't know, thin mini-itx is different from mini-itx, because, well, they are thinner. The easiest way to recognize a thin mini-itx mobo is to look at the ports on the back, if there are ports stacked higher than two USB ports, most probably they are not a thin mini-itx motherboard. And they usually have a DC-in connector port for external PSU.