There's a full trailer (not a teaser) available online now at
The fact that it's not on Pixar's site and the fuzzy quality of the compression leads one to believe that the release of this clip has not been authorized by Pixar. It's been available online since at least October though so they must not be too worried about it.
It's a lot longer than the two teasers available so far and reveals a lot of the plot points and additional characters.
Pixar's teasers never give you a good idea of what the movie will be like. The trailers do a better job, but ultimately Pixar movies are about the story so until the final movie is released we really won't know how good that story is. Given their trackrecord though, it should be good.