They are no way near as dominant as Microsoft was with windows, and they are not promoting their own product/ software with something to gain, but just some (other?) open standards on the web. What on earth are they going to sue for?!
The only thing I could see them realistically filing suit for -- i.e., anything that isn't just another temper tantrum to try to turn public opinion against Apple -- would be damages because of theoretical lost sales of CS5.
But as long as Apple can demonstrate that they told Adobe they would not be in support of this, they've got nothing. I'm not even sure that Apple would even need to prove that, as the developer agreement lays out that Apple can change whatever they want, whenever they want.
This rumor -- and any lawsuit, should it happen -- is a PR hit job, plain and simple.