That's what Game Informer are saying, having spoken to a number of developers at E3. They say none of the devs polled had any inkling the product was to be announced, and that the general feelings amongst them were those of "annoyance and betrayal". If that's true across the board (and it seems to be, as it echoes similar sentiments we heard at E3), it's good news for Nintendo's secret-keeping team! But bad news for third-party devs and consumers, because it means anyone who's not Nintendo is only now able to plan for games that make the most of the add-on.
Wii MotionPlus Took Developers By Surprise [Game Informer]