In MyFaces 1.2.5-SNAPSHOT, the type attribute of actionListener has been set to a non-required attribute. But if you have something like this in your page:
<h:commandButton value="This button with binding attribute">
It will throw a javax.servlet.jsp.JspException - type attribute not set. This also happens in MyFaces 1.2.4. I've attached the test case above.
This problem does not exist in JSF RI 1.2 though.