[рони]

Bezlepkin,
Вариант ...

<form method="post">
<input type="text"/>
<input type="submit"/>
</form>
<script type="text/javascript">
document.forms[0].onkeypress = function (a) {
    a = a || window.event;
    if (a.keyCode == 13 || a.which == 13)
    a.preventDefault ? a.preventDefault() : a.returnValue = false
};
</script>