There is a great deal of good and some bad information in this thread. Here's some facts you can take to the bank.
1. The front and rear drivetrains are completely separate - there is no mechanical connection between the two axles
2. Both front and rear drive motors can power the vehicle in either direction, forward or reverse
3. The rear electric drive is active every time you accelerate from a stop up to 20+ MPH and then shuts off as long as front and rear axle speeds are the same
4. Both front and rear differentials are of the "open" type with no limited slip mechanism - however, the ABS and traction control system can make up for much of this lack of limited-slip functionality
5. In reverse, the Highlander and Rav4 hybrids have much less front torque available than in forward motion - this is a limitation of the dual motor-generator system being driven by the ICE. Thus the rear motor always helps in reverse. The lower available torque due to the engine and MG2 working in opposite directions is what causes the Highlander hybrid to have a lower towing capacity than the non-hybrid - it can't back up a 5000 pound trailer on a steep hill.
All in all, the AWD-i system used on Toyota's hybrids is the best in the world IMHO. But it has its limitations.
20 The whole system, called Lexus Hybrid Drive (also Hybrid Synergy Drive), produces a maximum of 200 kW. Under normal driving conditions only the front motor and gasoline engine will be used. The rear motor will only be used under full-throttle acceleration or when the front wheels lose traction.