I think maybe they're using a visual to emphasize what Gandalf said at one point, that in some ways he IS Saruman the White--as Saruman the White should have been. I think making him look like him is part of that, plus he has to look different enough (and enough like Saruman) that our three highly intelligent, observant heroes would not recognize him without looking foolish.
I also think PJ is using comparison-contrast with Gandalf and Saruman--by making their appearance somewhat similar, you see how it is that they differ.
The only thing that I'm not sure about is the way Gandalf is speaking--I hope that we will still see the kind, personable part of his character. He seems a bit unapproachable in the preview, it'll be interesting to see how they hang on to some of Gandalf the Gray's personality. Maybe it will be the hobbits in particular that bring that out in him...
I pretty much expected him to be "all in white" like Saruman. I just didn't think that Gandalf would go for the limp, straight hairdo (wonder how long it'll take him to straighten out his beard.) As for our observant heroes, have any of them seen Saruman before? I know they expect him to be all in white and hooded (if memory serves...I don't have TTT with me today).
(Gandalf having a bad hair day?) :))