Ok this is kind of new, never noticed it before. WMP default skin is kind of boring so I searched around for a new skin, couldn't find a nice enough skin to my satisfaction. I was playing around and changing skins when I noticed a nice somewhat transparent skin appear suddenly. This is how I found this:
First I applied the default skin. http://i52.tinypic.com/evcmr5.jpg
2nd the default skin appears. http://i53.tinypic.com/znrkpj.jpg
3rd you click the 'switch to library mode' button on top right corner of the window, after going to library mode click 'switch to now playing' button on bottom right corner, you get the nice looking hidden windows skin voila! http://i53.tinypic.com/2f055c0.jpg
anyone knew about this?