The Gov't owns a piece of PBR. There is a reason I am not too concerned at this point in time:
PBR has to issue trillions of reals in debt to drill and exploit the finds unless they decide to let other majors do their dirty work.
Regardless, I don't think the gov't will shot itself in the foot by nationalization because they would be unable to secure financing if they fuck over current shareholders.
As we saw in Argentina, production drop by what? 90%? when they nationalized? All the US oil service companies refused to continue and bailed.
Now when the reserves have been developed and the money is flowing in I would have concern since they would be rolling in cash. Why ruin a good thing? Even though Brazil leans left I think they realize the result of taking the Chavez route!