Acording to the guy who has seen the first cut (that is if he really saw it) who reported on theforce.net, Anakin does turn into vader before he fights with Obi Wan. He also said that you see Palpatine turn into Siddeous right before your eyes, as a transformation, so I am assuming the eyes is not an effect for the preview and is actually what happens when someone turns to the dark side. We don't know what Anakins eyes looks like in Ep 4and 5 as he has a helmet on. When it is taken off at the end of 6, he has turned back to the good side before it is taken off, so his eyes may have looked like that up until he turned back again. We wont know anything until the thing comes out. But I highly doubt that Lucas would have put that shot in the preview just for effect. YOu don't see anything in the Prequel previews that doesn't show up in the actual film, so I am assuming that it is a shot from the actual film.
This isn't a "teaser" as teasers are previews that don't have anything from the film in it as the film hasn't been shot yet, or is being shot at that time the teaser is released. Teaser's don't have footage from the actual film; but they may have a few shots, and are just slapped together, usually by the studio's advertising department, to advertise the thing and to make audiences are aware that this thing is being shot now and will come out soon. That is why they are called 'teasers' and not previews. Causing they are teasing you by not showing you anything from the film. An example of what teasers are:
Jaws teaser: I think this is actually the first official teaser and that is why this one kind of breaks the rule and has one underwater shot of about 40 people in the ocean. But that is it. Nothing else from the film. This was put out by the studio as the movie went into production before the book that it was based on was released to the public. The book quickly became a New york Times best seller, and Universal wanted to capitolize on the books success and let people know that the movie was being shot now. The underwater shot may be a shot from production, or it may have been something that Spielberg new he was going to do in the film, so he shot it in L.A. before production headed to Marthas Vineyard.
Superman's teaser: Footage shot from a lear jet going through clouds with the actos names shooting into frame as they do in the opening credits, but that is it.
Close Encounters of the Third Kind's teaser: The camera going down the road with the glow at the end. This was a Greg Jein Miniature, and they used footage from it in later previews.
Alien: The Chicken egg on the clay set cracking and light coming out of it. Fox and Scott didn't want to show anything from the film, or the Alien, to add mystery and the previews are shorter cuts of the teaser. You could say that Alien never does have previews, and is only teasers.
Alien 3: Much like the Alien teaser, with an egg, but a different take on it.
Terminator 2: Shots of the terminator on an assembly line being manufactured.
Terminator 3: The liquid metal drops into blackness forming the logo "T" with the 3 cut into it.
Twister: Shots of a dusk sky and other stock storm shots edited together. At the end is the test shot that ILM did to show Warners they could do a CGI twister.
Godzilla 1998: This teaser is the one that takes place in the Museum of history. Nothing in it is in the film, and if you look, even the CGI model of Godzilla is different than it is in the film. That teaser was shot by Emmerich and team (since Emmerich created a EFX company to do Godzilla himself with people from the various companies that did ID4) to see if they could do everything they told Sony they could, and to see how the production team worked.
The Star Wars (A new hope and Empire) are the exceptions as those where released while the film was being made by George Lucas. If you look at the Empire Strikes Back teaser, it does have some shots from the film, but it is mostly production design paintings. What they do have in the teaser is some of the only things they had in the can at the time. It is stil a teaser, but mostly it is production art. A New Hope's preview (called a 'teaser' on the DVD) was mandated by the studio. But if you watch the documentary, Lucas says Fox needed the first preview, and they didn't have things done, and it had the only shots they could put into it.
The only guy who's previews were teasers was Hitchcock films, like Psycho's preview. It is him walking around the bates motel set, and in the Bates house, describing what will happen in the movie. That is just how Hitch liked his trailers to be. He knew you needed a trailer to advertise a film, but he didn't feel that people should see anything from the film.
So in short, this is a 'Preview' and not a 'Teaser'. Teasers may also have one or two shots from the film, or dialog, but that is about it. I have seen some DVD's calling the first preview 'teasers', and I don't know why that is. In fact some plces they are calling this a teaser, and I don't know why that is either. It is a preview.