I've just posted a bug report about this
    https://bugzilla.redhat.com/show_bug.cgi?id=694466
but thought I'd ask here, since someone else must surely have seen this.
Xelatex can't output math symbols when using fontspec. Even leaving the font
unchanged and loading fontspec doesn't help. This bug looks essentially like 
the one described here:

https://bugs.launchpad.net/ubuntu/+source/texlive-bin/+bug/364627

If you compile the test file in the bug report, the sum symbol does not display.

Unfortunately, the fix described here:

http://www.mail-archive.com/debian-bugs-dist@lists.debian.org/msg631117.html

does not work.

This is with fully up-to-date F14:

[rgheck@rghquad ~]$ rpm -q texlive-xetex
texlive-xetex-2007-56.fc14.x86_64