In the opening of "Patterns of Force", the Enterprise enters the system that is home to the planets Zeon and Ekos. On a mission to ascertain the fate of a Federation scientist observing the primitive and violent culture of Ekos, the Enterprise is attacked by a thermonuclear missile launched from Ekos, an impossibility based on what they knew of Ekos. Dispatching it with a phaser blast, the crew of the Enterprise discovers just what has happened on Ekos.
So what was so shocking to me?
Well, in that opening sequence, the Enterprise does a slow pass past Zeon, a blue-green world with a single bright ring. Canting slighlty, the ship heads for Ekos. When the missile is detected, Kirk orders it displayed on the main screen. The metallic missile with the bright rocket plume alters direction a couple of times as it locks onto the Enterprise. When the phasers fire, we watch from over the saucer section behind the bridge. The blue-green pulse of the phaser blast emerges from under the saucer and lances into space at the unseen target. There is a flash, and an expanding globe of light washes over the detailed duraluminum saucer plating before the scene returns to the bridge.
What the...?
OK, I had no idea that Star Trek was being remastered for High Definition. Apparently, every special effects shot from the original 60s program has been chucked and replaced entirely with CGI. Even the "people shots" have had the colour balance reworked and enhanced for presentation on modern TVs.
Here is shot from a different episode, "Errand of Mercy", highlighting the new look of the old Enterprise in action:

Interestingly, the end credits are not changed to list who is responsible for the new work, other than a banner added after the theme completes attributing this episode to CBS Paramount Television.
You can find out about the project on Trek Movie, and sample some captured videos from episodes already released.
I guess I haven't been paying attention to things Trek, so I was quite shocked at what I saw. But pleased. I'm looking forward to seeing more. If you've got HD channels from your cable or satellite provider, look for Star Trek on Saturday afternoons on ABC HE starting at 7pm.