From http://www.w3.org/MarkUp/html-spec/html-spec_8.html#SEC 8.2 under "Form Submission":

"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."

I found out about this when I found the page http://niwo.mnsys.org/saved/~flavell/www/formquestion.ht ml