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