The signature of AjaxHelper::submit() has changed, and is not compatible with code from RC4 (or before), i.e. it will break existing code. Here is the old function signature:

function submit ($name, $value, $options = array())

It has been replaced by the following function signature:

function submit ($title = 'Submit', $options = array())

You know the drill. Make sure you update your code if you use this function ;-)