2.8 AJax in my plugin

The plugin I wrote stopped working fully due to changes when upgrading to 2.8. More specifically the ajax stuff. All requests to admin-ajax.php execute successfully on the php side but the response is never communicated to the javascript.

I’m using jquery so if I do something like

Query.ajax({ url:"admin-ajax.php", data:{action:'myplugin_update', data:'blah'}, type:"POST", dataType:"html",
error:function(XMLHttpRequest, textStatus, errorThrown {alert(errorThrown); },
success:function(data,textStatus){alert(data);} });

neither error nor data alerts are triggered but all php code associated with the ‘wp_ajax_myplugin_update’ action are executed successfully.

I would expect to at least get the error but right now I’m stuck as to what I should check. It effects both IE and FF