I finally worked out why my forms weren't submitting when the user
hits the ENTER key. lowpro serializes the button that was clicked
along with any other parmeters when submitting a form via AJAX.
However, when the user hits enter under FireFox, there is no button
and the consequently the browser barfs. Safari on the other hand tries
to be too helpful and triggers an onclick event for the first submit
button (which is why I never noticed it).

So anyway, rather than try to be too clever myself, I simply changed
the parameter serialization in Remote.Form.onsubmit to look like:

parameters : this.element.serialize({ submit: this._submitButton ?
this._submitButton.name : null })

Problem solved.