Per MLB Network, Game 6 has been postponed until tomorrow. Game 7, if needed, will be on Friday.
As Adam said:
Ron Washington has already said that Colby Lewis will start Game 6, whether that's today or tomorrow, and Matt Harrison will start Game 7. After Harrison's shaky Game 3 start and Derek Holland's terrific Game 4 start, there was some thought that Holland could go in Game 7 if rain postponed things by a day, but Wash is saying it will be Harrison regardless.