According to an interview by GoNintendo this week, Rage of the Gladiator will be the first WiiWare game to offer MotionPlus support.
Here is an excerpt from the interview:
Ed Roman "We listened to you, the Wii community. The community encouraged us to explore Wii MotionPlus and we listened to your feedback. The result is pretty awesome – we actually love playing it with Wii MotionPlus even more than traditional controls. We've also added a basic motion sensing mode as well, for people to use the Wii Remote + Nunchuk in case they don't have a Wii MotionPlus device."