Using WordPress 3.5 Media Uploader in meta box?

To get you started, the basic functions and overrides as far as I know currently.There might be better solutions, but I only had two days with 3.5 yet:

// open modal - bind this to your button
    if ( typeof wp !== 'undefined' && wp.media && wp.media.editor )
        wp.media.editor.open( ##unique_id_here## );

// backup of original send function
   original_send = wp.media.editor.send.attachment;

// new send function
   wp.media.editor.send.attachment = function( a, b) {
       console.log(b); // b has all informations about the attachment
       // or whatever you want to do with the data at this point
       // original function makes an ajax call to retrieve the image html tag and does a little more
    };

// wp.media.send.to.editor will automatically trigger window.send_to_editor for backwards compatibility

// backup original window.send_to_editor
   window.original_send_to_editor = window.send_to_editor; 

// override window.send_to_editor
   window.send_to_editor = function(html) {
       // html argument might not be useful in this case
       // use the data from var b (attachment) here to make your own ajax call or use data from b and send it back to your defined input fields etc.
   }

This is not complete working answer. You have to define and keep track of your input fields by yourself etc.. This just should get you started.
If you have more concrete questions, just ask.

And make sure to reassign the original functions when your script is done.

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_{Pulled from comments:}

How can I listen to the close event of the lightbox?

// add listener: 
wp.media.view.Modal.prototype.on('close', function(){ console.log('triggered close'); }