Ubisoft's Marie-Sol Beaudry was recently interviewed for the Nintendo Europe website about her work on the upcoming Wii slasher/shooter, Red Steel.
During the interview, Marie-Sol was asked how current development was going, she replied with "We just discovered the built-in controller speaker feature around E3 and we are now working on how to use it for single and multiplayer.".
Obviously Nintendo decided to keep the Wiimote speaker quiet even from third party developers, I guess they were concerned about certain first parties getting wind of such ideas. Ahem.
The speaker is embedded into the actual Wiimote, Nintendo demonstrated this with Wii Sports: Tennis. As you swing and hit the ball, your remote will emit the sounds, almost like a dynamic surround sound. Hopefully we'll see more and more third parties taking advantage of this, yet another unique feature to Wii.
We'll have more as it develops