The poster isn't black and white, (it even says so at http://spiderman.sonypictures.com/.)
So if you are familiar with the Spider-Man lore, it pretty much confirms that one of the villians will be Venom in the third movie.
However, could it also be a broad arching set up for the Venom storyline in a possible fourth movie?
Hard to say, but I'm leaning heavily towards Venom being in the third movie.
Spider-Man 3 Teaser Poster