Please add it.
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Mathoverflow is an SE 1.0 site. It has been a great success, and will therefore at some point be ported to SE 2.0 and be included in the Stack Exchange family that is listed on http://stackexchange.com/sites. Yet some work needs to be done - like integration of LaTex in SE 2.0 for certain sites, or better, adding a site specific plug-in system, and replacing all the meta questions back to a compatible site, as it seems.
There are other SE 1.0 sites out there that aren't listed, like answers.onstartup.com - quite successful - and god.stackexchange.com - needs resurrection, which is done in multiple proposals on Area 51 like this one.
So it is only a matter of time for the site to be ported and included.