The keyboard accessory we were looking forward to in Rock Band 3 is supposedly actually a KEYTAR. Good, I suppose, for mobility and rockitude, but not for learning how to actually play the keybo...oh god I've become an old man. [Ars Technica]
Not any better for learning to play than one of the "guitars" for learning to play a real guitar.
As a novice picker, I find Guitar Hero/Rock Band to be very detrimental to my actual playing when I switch to the real deal.
Although the drums are actually a bit of a boost to beginner drummers, especially helping with timing and movement.