It looks like Google isn't wasting any time laying down a support infrastructure for its new set-top box push. Just one day after the shiny new Nexus Player officially went on sale, the YouTube app for Android TV has been published in the Play Store to enable easy updates without a firmware upgrade. If you happen to be one of the few people with an ADT-1 developer unit from Google I/O or you got lucky with an early delivery of the Nexus Player, you should see the update automatically.
As far as the actual update is concerned, it doesn't add much: YouTube video recommendations have been added to the homescreen, but based on the screenshots in our own Nexus Player review, they seem to be there already. And YouTube results will now appear in the system-wide search, which might be useful for those totally legal uploads of re-dubbed Japanese cartoons. If you're still waiting on your Nexus Player to arrive (which, as far as we can tell, is almost everyone who ordered one so far), expect to see an automatic download when you sign in with your Google account.
Naturally you can't download the app from the separate Play Store listing on anything except an Android TV device. Expect updates to both the app and platform as Google hammers out the initial bugs.