As I follow Toyota closer than any other F1 team, then I have got such kind of an impression about them that if they leave F1, then they'll leave altogether. I don't see much point of becoming an engine supplier as the development is frozen and any advantage can't be found there anyway. Toyota's engine chief Marmorini has left and the team itself has been critical about the engine rules, having threatened to leave F1 for Le Mans, where they would have more freedom. And I suspect that if not after 2009, then at latest after 2010 they'll indeed make that switch.
For Williams the most probable engine choice from 2010 onwards is Cosworth, so I guess 2009 will be the last year for Nakajima in F1 too.
It wouldn't really cost them much to keep supply engines that don't have to be developed. Furthermore, though Marmoni is credited with all the engine wok, a lot of it is done at the Fuji Technical Center in Japan. So I really don't see why they wouldn't take advantage of something they already have to keep their name in (or introduce Lexus' into) the sport.
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