Certain elements (such as grqq{} quotes or decimals from the dcolumn package) are missing when rendering with TexShop and XeTeX on Mac OS X 10.6.1 (Snow Leopard). Switching to the dvipdfmx engine did not help, it only misses other characters. My TexShop version is 2.26 (latest atm. I believe).
The problem persists for the couple of fonts I tried. Prior to Snow Leopard I had no problems at all. Any ideas / suggestions?
I believe that this problem will be fixed in MacTeX 2009, to be released soon. You can download a pre-release version here, if you're game:
http://www.tug.org/texlive/pretest.html