to add the enter support its simple.
13 is the keycode for enter.
to add the enter to submit the form its quite simple to do just make this little modification to ash's script.
inside ash's script we would add something like this on about line 35.
code:
document.forms.namedItem('gmsearch').addEventListener('keypress',function(e){if(e.keyCode==13){test();}},false);