A report about BMW R&D on the Motor Authority website attributes a "real world" gain of 5% in fuel economy to use of a thermoelectric generator to produce electricity from exhaust heat.  As much as 1 kW of electricity could be produced this way.  The US DoE FreedomCar program goal is 10% fuel savings.

The report also indicates a Rankine Cycle project attributed to Honda has produced up to 32 kW, but a weight penalty reduces the fuel efficiency gains to 3.8%.   Assuming the Motor Authority is referring to the same Honda project discussed in this GreenCarCongress.com article, the 3.8% refers to an increase in engine thermal efficiency, which isn't quite the same thing.  The GreenCarCongress.com article also indicated Honda wanted to see higher efficiencies before putting the Rankine into production.