YouTube Remote for Android -- control Google TV or Leanback from your phone
Unfortunately, as the app is only available on the American Android Market, I'm unable to test it. From the screenshot, it looks like the interface is fairly simple: you can skip, stop, and pause videos, change the volume, and add new videos to the playlist. I can see this being more useful for Google TV where you're actually some distance from your keyboard -- if you're using Leanback on your PC, it's just a matter of leaning forward to hit the pause button, or skip to the next video.
This is a beta app, incidentally. The official YouTube Android app will almost certainly be updated to contain the same Leanback connectivity in due course. The QR code is after the break.