My understanding is this:
WP8: you can connect and sync using Xbox Music, etc. I don't exactly know how this works but plan to find out shortly; getting the Nokia 920 today.
WP7.5: you'll still need to use Zune software, so no RT support.
WP7.8: should follow the new paradigm as WP8. The client software/update has not yet been released but should coincide with the release of 7.8.