Generally, browsers will map the <ENTER> key to emulate a mouseclick for whatever page element currently has focus. Most browsers are smart enough to emulate the "SUBMIT" for a form <i>if</i> the cursor is currently in one of the form's fields. The exception to this is if the cursor is in a field that allows CR/LF's (like a text area), or if you have javascript event handlers on the form fields.
If your sure that your form doesn't have any of these exceptions, post a pointer to the page and I will take a look at it.
-- Rich