Which mfg claims they "accelerate the outer rear wheel during cornering for improved handling"?
 
The outer wheels on my Ford pickup accelerate when cornering too, lol.  What is Audi(?) doing differently, braking the inner one?

The audi quattro system uses torsen diffs IIRC.

Subaru uses an active center diff on some models, it does communicate with the "VDC" system.

Teh WRC is likely the state of the art when it comes to active drivetrain elements, but usually the diffs are used since that wastes less energy.

I have heard of some automakers whose AWD technology relies on open differentials and uses the brakes to provide some sort of wheelspin management. (IE: a standard open diff transfers all the torqe to the wheel with the least grip. When the ECU senses one wheel spinning out of control the brakes are applied to that wheel in order to transfer torque back to the wheel with traction. I imagine that this system could be used rather effectively with the right amount of tuning.)

While I wrote that I thought 4-matic maybe?

Most ESP/ASR systems incorporate electronic diff locks as described by NickE which use the brakes.  Try looking at VW/Audi products to see if you can find anything related to those systems.

The Nissan Skyline and Mitsubishi Lancer Evo's have been using electronic yaw control for a while so you might to look at these a little closer for further info on the systems.

Most of the automotive system regulate the torque sent out of each side of the differential, not the speed, and typically it's the center diff regulating the torque between the front and rear.  For example, in the dry they'll send 90% of the torque to the rear wheels and only 10% to the front, but in the wet they'll split it 50/50.  Honda's SH-AWD (Acura RL) varies the torque split between the rear sheels to help the car to yaw and consequently the speed of the inner and outer wheels to differ.  They don't use the vehicle brakes to do this, but have some type of electromagnetic clutch system in the diff.  You can read about it here: http://www.auto-report.net/index.html?sh-awd.html  I don't know if the Skyline and Evo do this or if they just use the brakes as most cars with dynamic stability control or yaw control do; Honda implies they're the first to contol the torque split between wheels.

Anyways, getting back to the original question about varying the speed, are you trying to vary the speed independently (you start at 500 RPM Left, 500 right, then go to 500 left, 600 right) or have them coupled (500 left, 500 right, then go to 600 left 400 right)?  If the latter, a google search turns up tonnes of info on active diffs that will give you info, though probably not as technical as you're looking for.  While its all about varying the torque, they do use the wheel speeds to determine the amount of torque split to use.  If you're looking for the former, a diff won't do that job, unless you're also planning on controlling the input RPM.