Speculation has the hybrid version using the powertrain from the 600h sedan - a 5-liter V8 with a continuously-variable transmission and an electric motor/battery auxiliary power unit.
At the same time, the gasoline version will follow Toyota's Formula 1 cars in switching from the previously rumored V10 to a V8.
Related factoids for your consideration:
- F1 rules for 2008 and beyond are considering regenerative braking systems (and longer-term, alternative powertrains)
- Lexus recently completed a 24 hour endurance race with a modified GS450h - the first hybrid car ever to compete in an FIA-sanctioned circuit race