After some research, I found this a bit.
http://code.google.com/p/phantomjs/issues/detail?id=247
It seems SVG works best.
One problem that I encountered when testing the screenshot again after using the SVG version of the font was WebKit caching an old page and still showing broken fonts. Make sure your server explicitly indicates that it is not using the cached version, or clear the WebKits cache.
Brad
source share