According to 9to5Mac, Hulu Plus is ready to be deployed on the Apple TV, but Apple has yet to green light the implementation:

While there are no technical issues standing in the way of the Hulu Plus release on Apple TV, there appear to be some political ones. At some level at Apple, there appears to be some consideration that the Hulu Plus app could eat into iTunes TV sales on the Apple TV. Where Netflix tends to run older programming, iTunes is the Apple TV’s only outlet for current TV programming.

There is a workaround: iPad 2 and iPhone 4S users running iOS 5 can use AirPlay to mirror the Hulu Plus app content on their 2nd generation Apple TVs. One limitation of this method is only standard definition (SD)—not high definition (HD)—video will be mirrored to your television.

As for content licensing, I wouldn’t be surprised to find that only part of the web-accessible catalog is available on the Apple TV, much like what we have seen with the iOS apps and PS3. You may see this disclaimer more often than you would like to admit:

We currently don’t have the rights to make this show available on TV or mobile devices — request to be notified if it becomes available in Hulu Plus.