I don't know...
iTunes doesn't have one. I don't really know that it's needed (especially if there's some other tooltip-explained method for stopping anyway). If it makes the interface a little cleaner, then I'm all for it (as long as we don't lose functionality). Again, because I'm stuck at work I haven't seen it yet (just a few more hours)...
However, we do still need to be able to access the Stop function through the command line and windows message interface. My remotes already have a stop button afterall!