Z6PO said:(remember, in episode IV, he doesn't know the princess when R2 plays the hologram message, but earlier in the movie he says "there will be no escape for the princess"... I wonder if he may have some memory malfunction...)
On the contrary, Threepio remembers everything. Perfectly. But he is a protocol droid, in the service of the Royal Family of Alderaan, and it would be against his programming to disclose their secrets to common farmboys. Or the secrets of his previous masters. It just wouldn't be proper.