This is actually expected behaviour in some cases.  If the form has a single input text field then by w3c html 2.0 spec it should submit on enter.

see http://www.w3.org/MarkUp/html-spec/html-spec_toc.html

specifically:
  "When there is only one single-line text input field in a form, the user agent should accept Enter in that field as a request to submit the form. "

If you have a textarea in your form than it wont work.  This behaviour also changes depending on the browser.  For instance netscape allows for textareas.

I ran a quick test that was a form with two text-Box controls and one button.  I was able to submit with enter in all cases.