The movie: Everything from fire to tears floats in the zero-g environment of the ISS, the Soyuz, and the Shenzhou. But Bullock's hairdo is always nice and coiffed.
Real life: As evidenced by this epic picture of astronaut Marsha Ivins, hair does indeed obey the laws of physics. The movie is already so consistent with the zero-g environment that the hair thing can be forgiven. But it was a little weird when Stone's helmet was still floating around during the re-entry scene. It was long past time that objects like that let gravity literally put them in their place.