[ваый]

<input type="text" class="test">

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js"></script>
<script>
$('.test').keypress(function(e) {
    if (e.keyCode < 48 || e.keyCode > 57) {
        return false;
    } 
});
</script>