While Skyhammer may not be officially listed as a character on the IMDb or Transformers 3 Wiki page that just means Paramount may not have a voice actor associated with the character. There were several robots in Transformers 2 who both appeared in the film, and were represented in the official toy line, who didn't have a voice or show up in the credits. The scenes from the trailer where military personal jump out of a helicopter wearing "wing suits" could certainly be Skyhammer.
Just like Skyhammer, Powerglide isn't listed on IMDB but with a band of Transformers waking from their deep space slumber on the moon - who knows what other new characters fans are bound to see on screen.
It's was confirmed early on that Shockwave would be the main villain in Transformers 3. However, like Megatron, Shockwave was a gun in the original comics and needed a hefty make-over for the film. Bay went with a more believable tank mode and while the on-screen version appears to be a sleek shiny metallic color, the toy sports the purple Decepticon color fans know so well.
Since last fall, fans have known that the mighty Sentinel Prime would make an appreance in this film as a Rosenbauer Panther 6x6 firetruck but what part he will play in the overall story is still a mystery - though if they let him chop up some Decepticons with that massive double-sword then fans may not care.