Originally created by @osvaldasvalutis, callback support by @printempw.

example.js

$.disqusLoader('.disqus', {
    scriptUrl: '//blessing-studio.disqus.com/embed.js',
    laziness: 0,
    preLoadCallback: function () { $('.disqus-notice').show() },
    disqusConfig: function () {
        this.callbacks.onReady = [function () {
            $('.disqus-notice').hide()
        }];
    }
});

jquery.disqusloader.js

/*
    disqusLoader.js v1.0
    A JavaScript plugin for lazy-loading Disqus comments widget.
    -
    By Osvaldas Valutis, www.osvaldas.info
    Available for use under the MIT License
*/

;( function( $, window, document, undefined )
{
    'use strict';

    var $win             = $( window ),
        throttle         = function(a,b){var c,d;return function(){var e=this,f=arguments,g=+new Date;c&&g<c+a?(clearTimeout(d),d=setTimeout(function(){c=g,b.apply(e,f)},a)):(c=g,b.apply(e,f))}},

        throttleTO       = false,
        laziness         = false,
        disqusConfig     = false,
        scriptUrl        = false,
        preLoadCallback  = false,
        postLoadcallback = false,

        scriptStatus     = 'unloaded',
        $instance        = $(),

        init = function()
        {
            if( !$instance.length || $instance.data( 'disqusLoaderStatus' ) == 'loaded' )
                return true;

            var winST = $win.scrollTop();

            // if the element is too far below || too far above
            if( $instance.offset().top - winST > $win.height() * laziness || winST - $instance.offset().top - $instance.outerHeight() - ( $win.height() * laziness ) > 0 )
                return true;

            $( '#disqus_thread' ).removeAttr( 'id' );
            $instance.attr( 'id', 'disqus_thread' ).data( 'disqusLoaderStatus', 'loaded' );

            if( scriptStatus == 'loaded' )
            {
                DISQUS.reset({ reload: true, config: disqusConfig });
            }
            else // unloaded | loading
            {
                window.disqus_config = disqusConfig;
                if( scriptStatus == 'unloaded' )
                {
                    scriptStatus = 'loading';

                    if (typeof(preLoadCallback) == 'function')
                        preLoadCallback();

                    $.ajax(
                    {
                        url:        scriptUrl,
                        async:      true,
                        cache:      true,
                        dataType:   'script',
                        success:    function()
                        {
                            scriptStatus = 'loaded';

                            if (typeof(postLoadcallback) == 'function')
                                postLoadcallback();
                        }
                    });
                }
            }
        };

    $win.on( 'scroll resize', throttle(throttleTO, init));

    $.disqusLoader = function( element, options )
    {
        options = $.extend({},
        {
            laziness:         1,
            throttle:         250,
            scriptUrl:        false,
            disqusConfig:     false,
            preLoadCallback:  false,
            postLoadcallback: false,

        }, options );

        laziness         = options.laziness + 1;
        throttleTO       = options.throttle;
        preLoadCallback  = options.preLoadCallback;
        postLoadcallback = options.postLoadcallback;
        disqusConfig     = options.disqusConfig;
        scriptUrl        = scriptUrl === false ? options.scriptUrl : scriptUrl; // set it only once
        $instance        = ( typeof element == 'string' ? $(element) : element ).eq( 0 );

        $instance.data( 'disqusLoaderStatus', 'unloaded' );

        init();
    };

})( jQuery, window, document );