I am dying for someone to post a teardown or interface about this new wii motion plus dongle. Ive played with the nunchuck and it was all real easy to interface with the arduino so I want to see if the wiimotion plus is equally easy (before i go and buy one).
The main thing I am worried about is that the gyros are 1500deg/s and Im not sure how well the ADCs in the arduino are gonna be able to read small rotation movements. If anyone has one to play with, and a little time to modify (may not be needed) the nunchuck code, please post how sensitive a reading you can get from the arduino on it.
It could be the first super cheap super available imu when paired with the nunchuck (40$)
I thought the wii motion devices used I2C to send data? If true then the data would not go through the Arduino's 10bit A/D convertors, it stays in the digital domain the whole way.
Lefty
Yeah, I figured out my dumb mistake a few days after I posted this. Amazing how long some things can take to sink in. Im having a "care package" delivered in a few days and might take a stab at all of this myself. Ive been trying to read up around the internet about the wii accessory port and have seen that direct i2c protocol doesnt change between peripheral devices too much so the work already done with the nunchuck should be very helpful.
The biggest snag I see is that the wiimotion plus itself has an expansion port on it meaning that it must be at some other i2c address than 0x52 (right?). Oh yeah and the fact that once I(or someone else...hint hint) get data from that, I wont have any idea what that means. But we'll cross that bridge once we come to it.
Tada!
http://wiibrew.org/wiki/Wiimote/Extension_Controllers#Wii_Motion_Plus
16 bit resolution and 3 axis! Even better than I was hoping for. Eventually I'll get around to writing arduino code for it, but I'm pretty sure someone else could do a better, quicker job of it 
thanks xandar, that will prove to be really helpful. I actually saw that a few days ago and since then its been updated.
In other news...I found the I2C Address of the Wii motion plus! Using a really simple program to ask every possible I2c address for info, I found 15 responses with the wm+ hooked up same as nunchuck(or any other wii periph) would be. Using this page: http://www.esacademy.com/faq/i2c/general/i2cexcep.htm I eliminated all of special reserved addresses (14 on a 128 node bus) and found that the i2c address of the wii motion plus is 83 or 0x53 (just so happens to be one more than the address of the normal periph slot)
Next step is finding the initialization sequence! Yay!
Summary: Wii motion plus I2C address =83 or 0x53
Hi !
I'm new here and i'm french so, if i make a few faults correct them! 
I've looking for some information about the Wii Motion Plus to communicate with. The most post use the wiimote but not directly the wm+. Great job to have found the 0x53 address. I think we can see the i2c signals between wiimote & wm+ and read registers configurations with a scope ...
knuckles, can you still talk to the nunchuck at it's normal address if it is plugged into the m+?
sorry, someone else will have to answer that. My wiimote and nunchuck are at my school apartment right now. Im home from college for the summer. I would assume so, otherwise using an i2c bus would be worthless
Xandar,
If your m+ is plug in to your wiimote, the adress is 0x55 i think : Wiimote/Extension Controllers - WiiBrew.
but you pass by the internal cpu of the wiimote.
Ok Im done. Got the WM+ working on my arduino. Everything seems stable. Check it out here: http://randomhacksofboredom.blogspot.com/
Gonna post in exhibition too
BTW: blogger sucks to post code in