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.
"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.