This is the big question.
Not without an update, I'd think. Will Apple allow it?
I keep waiting for the AppleTV to get is own app store. I think it's likely you'd see it available then just like you see Hulu+ and Netflix in the iPad store.
But without a store will Apple add this and ONLY this in an update soon? I sure hope so, but I'm not too hopeful.
If Apple incorporated Netflix into the ATV2 despite what every one said about them not doing it (remember when that rumor first started circulating) because Netflix would compete against iTunes, I don't think they'll care too much about a YouTube that supports video rentals on ATV2 or the iOS devices for that matter. I'm betting that app support for ATV2 is something we'll see soon and that some of those apps (Netflix, Hulu, YouTube, GoogleTV, Pandora, Rdio, etc.) could compete directly with iTunes