I kind of agree and disagree with Nomuru at the same time here. On the one hand, Pinkie's behavior was frustrating and illogical(not that we've come to expect logic from her, but hey), and yeah, the plot as it was would have been resolved if she had told one or the other, but on the other hand, I don't think Twilight should have told them. They more than likely would have forgiven her, or even thanked her for telling them, but it was their problem, and their choice to solve it. I think that the plot would have been perfectly fine if Twilight had been debating the issue on her own - it was Pinkie's incessant insistence that the secret must be kept at all cost that lessened it. Really, I think that Pinkie was acting pretty harshly out of character here - she embodies laughter, after all - you know, fun? It was obvious that her behavior was directly or indirectly making three of her closest friends absolutely miserable, and could easily have lead to a serious conflict between Rarity and Fluttershy. It may have simply been a case of poor communication between her and Twilight, but that's still a baaad Pinkie Pie.
Readers Digest version: Pinkie's behavior and insistence that Twilight not reveal the secret was unacceptable, but Twilight shouldn't have told the secret anyways. Hint, maybe, nudge the two towards it, sure, but ultimately it was something that they needed to do on their own.
That aside, I'm actually rather taken with the episode. Rarity's little fits are always kinda fun, and after reading a couple of comments that I thought were sarcastic, I was worried that Fluttershy was going to get a swollen head and become a total primadonna, but she stayed her lovely self the whole way through. Yay. Not to mention Photo Finish's complete hamminess.