At the risk of sounding too negative, it seems like this was probably not a voluntary departure.

Yahoo's home page still suffers from ADD overload, Flickr's UX has been mangled, and Yahoo Mail is underwhelming, at best.

Add in the fact that the All Things Digital article mentions Marissa Mayer's "hands-on" approach to design, and it sounds like what happened here was a clash of personalities, possibly tying Tom Parsey's hands and resulting in a series of designs that no-one was happy with, including Parsey or Mayer.

It will be interesting to see who replaces him, and whether she or he is allowed to have a long or short leash, publicly.