This function creates a temporary form, then send data using jQuery :
function postToIframe(data,url,target){
$('body').append('<form action="'+url+'" method="post" target="'+target+'" id="postToIframe"></form>');
$.each(data,function(n,v){
$('#postToIframe').append('<input type="hidden" name="'+n+'" value="'+v+'" />');
});
$('#postToIframe').submit().remove();
}
target is the 'name' attr of the target iFrame, and data is a JS object :
data={last_name:'Smith',first_name:'John'}