I upgraded last Friday, and all of my Zune Pass content showed up in Xbox Music and plays.
The only thing I had to do is un-hide all of my folders and files under the my music folder. For some reason everything in it got changed to hidden when I upgraded. After doing that all of my music shows up and plays. To do this just select the My Music folder and change it to hidden (it itself was not hidden), applying to all subfolders and files. Then when it's done, do the opposite, un-checking hidden, applying to all folders and files.
Maybe you won't have this same problem though.