Edit: This question gave me an itch, so I put up a JSONP webservice on Google App Engine that returns the clients ip address. Usage:
<script type="application/javascript">
function getip(json){
alert(json.ip); // alerts the ip address
}
</script>
<script type="application/javascript" src="http://jsonip.appspot.com/?callback=getip"> </script>
Yay, no server proxies needed.
Pure JS can't. If you have a server script under the same domain that prints it out you could send a XMLHttpRequest to read it.
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