mboxDefine() and mboxUpdate() are supported by at.js. All existing mboxDefine()/mboxUpdate()'s on your site should continue to work! A few things to keep in mind:

1. mboxUpdate() now uses the json endpoint instead of the "ajax" endpoint. Because of this debugging is a little different.
2. mboxUpdate()'s invoked during page load now fire immediately. Previously they waited until window.onload.
3. window.targetPageParamsAll() will also populate parameters in an mboxUpdate() call.