In tapestry.js, line 808, inputs type submit are ignored for form serialization (in case of ajax form submission). But button type submit are also a valid way of submitting the form. Besides, as with input type submit, it doesn't make sense to send buttons as fields values.
Prototype 1.6 automatically ignored button elements when serializing, but 1.7 includes them. In order to avoid sending this data pointlessly, line 808 could be changed to:
return (e.tagName == "INPUT" || e.tagName == "BUTTON") && e.type == "submit";