From Dynamic HTML: The Definitive Reference, page 264:

Default behavior of the Enter key in forms has evolved into a recognized standard. When a form consists of a single text INPUT element, a press of the Enter (or Return) key automatically submits the form (as if the user had clicked on a SUBMIT button element). If the form consists of two or more text INPUT elements, the Enter (or Return) key does not automatically submit the form.

What is left unstatated is when the behavior evolved into a recognized standard. YMMV with older browsers.