See http://docs.mathjax.org/en/latest/advanced/typeset.html:

If you are writing a dynamic web page
where content containing mathematics
may appear after MathJax has already
typeset the rest of the page, then you
will need to tell MathJax to look for
mathematics in the page again when
that new content is produced. To do
that, you need to use the
MathJax.Hub.Typeset() method. This
will cause the preprocessors (if any
were loaded) to run over the page
again, and then MathJax will look for
unprocessed mathematics on the page
and typeset it, leaving unchanged any
math that has already been typeset.

You should not simply call this method directly, however. [You should instead] queue the typeset action, [using this] command:

MathJax.Hub.Queue([Typeset,MathJax.Hub]);

Demo here: http://cdn.mathjax.org/mathjax/latest/test/sample-dynamic.html