$.getJSON(url,
function(data){
$.each(data.items, function(i,item){
$('#testfield').html('<p>' + item.d.math_text + '</p>');
});
MathJax.Hub.Queue(["Typeset",MathJax.Hub,"testfield"]);
});
See http://www.mathjax.org/docs/1.1/typeset.html for more details.
Davide