Textfields can't be multi-line, so that option is out. Intercepting a button press is complicated across browsers. jQuery provides a standardised interface into this data (if you're not using jQuery this will be difficult).
Extract from Form submitting when pressing enter in Textarea
$('#myTextArea').keydown(function(event) {
if (event.keyCode == 13) {
$(this.form).submit()
return false;
}
}