Edit in JSFiddle

JavaScript
HTML
CSS
Result
Resources

/*
    jQuery Long Poll plugin 0.0.1 (02-05-2010)
    
    Note: requires jQuery-JSONP: http://code.google.com/p/jquery-jsonp/

    Copyright (c) 2010 JavaScript Guy

    This script is licensed as free software under the terms of the
    LGPL License: http://www.opensource.org/licenses/lgpl-license.php
*/
(function($){
    
    $.longPoll = function( args ) {
        args = args || {};
        var self = $.extend( {
            url: '',                //    URL To connect to
            tsParameter: 'ts',        //    Parameter used for the timestamp
            reconnect: true,        //    If we're automatically reconnecting
            errorTimeout: 60,        //    Timeout for errors, eg: server timeout, execution time limit, etc.
            errorTimeoutDelay: 5,    //    Seconds to delay, if the connection failed in an error
            hasConnection: false,    //    If we have a connection
            timestamp: 0,
            error: false,
            request: null,
            reconnectFunc: function() {
                if( self.reconnect ) {
                    self.disconnect();
                    //    Reconnect - with longer error timeout delay (in case we have an intermittent connection)
                    if( self.error ) {
                        setTimeout( function() { self.connect(); }, self.errorTimeoutDelay * 1000 );
                    } else {
                        setTimeout( function() { self.connect(); }, 100 );
                    }
                }
            },
            connect: function() {
                self.reconnect = true;
                //    self.request = $.ajax( {
                self.request = $.jsonp( {
                    url: self.url + '?' + self.tsParameter + '=' + self.timestamp + '&callback=?',
                    timeout: self.errorTimeout * 1000,        //    Timeout and reconnect
                    error: function(XHR, textStatus, errorThrown) {
                        self.reconnectFunc();
                        self.error = true;
                    },
                    dataType: "jsonp",
                    success: function( data ) {
                        self.timestamp = (data[self.tsParameter])? data[self.tsParameter]: self.timestamp;
                        self.success( data );
                        self.error = false;
                    },
                    complete: function( data ) {
                        self.reconnectFunc();
                    }
                    
                } );
                self.hasConnection = true;
            },
            
            // Aborts request
            disconnect: function() {
                self.reconnect = false;
                if( self.request )self.request.abort();
                self.hasConnection = false;
            },
            
            // User defined
            success: function( data ) {}
            
        }, args );
        return self;
    };

})(jQuery);

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>JSONP Long Poll chat demo</title>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
</head>
<body>

<div id="content"></div>

<p>
    <input type="button" name="connect" id="connect" value="Connect"/>
</p>
<p>
    <input type="text" name="message" id="message" value="" />
    <input type="button" name="sendMsg" id="sendMsg" value="Send"/>
</p>

<!--<script type="text/javascript" src="jquery-1.4.2.js"></script>-->
<!--<script type="text/javascript" src="jquery-jsonp-2.0.2.js"></script>-->
<script type="text/javascript" src="http://jquery-jsonp.googlecode.com/svn/trunk/core/jquery.jsonp.js"></script>
<!--<script type="text/javascript" src="jquery-longpoll-0.0.1.js"></script>-->

<script type="text/javascript">
//    Chat app
var chatApp = function( url ) {
    
    var self = {
        connection: $.longPoll( {
            url: url,
            success: function( data ) {
                // Ignore server timeouts, the script will handle it.
                if( data['data'].indexOf( "Maximum execution time of" ) == -1 ) {
                    jQuery('#content').append( '<div>' + data['data'] + '</div>' );
                }
            }
        } ),
        
        //    Sends a chat message
        sendChat: function(request) {
            jQuery.ajax( {
                url: url, 
                type: "GET",
                dataType: "jsonp",
                data: { 'msg' : request }
            } );
        },
        
        //    Toggle connection button
        toggleConnect: function( button ) {
            if( self.connection.hasConnection ) {
                self.connection.disconnect();
                jQuery( button ).val("Connect");
            } else {
                self.connection.connect();
                jQuery( button ).val("Disconnect");
            }
        }
    };
    
    return self;
};
</script>

<script>
    //    Chat app initialisation
    var chatAppContext = chatApp( 'http://paxjs.com/labs/longpolljsonp/chat.php' );
    //var chatAppContext = chatApp( 'chat.php' );

    var sendMessage = function() {
        chatAppContext.sendChat( jQuery('#message').val() );
        jQuery('#message').val('');
    };

    jQuery( '#connect' ).click( function() {
        chatAppContext.toggleConnect( jQuery( this ).get(0) );
    } );
    
    jQuery( '#sendMsg' ).click( sendMessage );
    jQuery( '#message').keyup(function(e) {
        if(e.keyCode == 13)sendMessage();
    });
</script>

</body>
</html>

External resources loaded into this fiddle:

jquery.jsonp.js