Is it ever really made clear that Andy is innocent? People always talk about this film in terms of Andy being wrongfully incarcerated, but I've always thought that his guilt or innocence was left intentionally vague. Yes, you have the prisoner who claims to know who really committed the crime, but this is a convicted felon we're talking about here, someone who could easily fabricate a story just to get a deal with the DA. And because he's killed, we never know for sure whether he was on the level or not.
Of course, Andy isn't really the main character in the film anyway. He's a catalyst for change, most significantly in Red, whose redemption is what the title refers to. In a way, though, if Andy did commit the crime, then he is redeemed to a certain extent too because of his acts while behind bars. I don't think those acts make up for killing two people, but it's a minor redemption of sorts.
RE the idea that the ending isn't real: I'd never considered that, and it's certainly a valid interpretation. Like the ending of Taxi Driver, which some argue is going through DeNiro's character's head as he dies, the ending of Shawshank could certainly be Red's daydream.
Does Darabont discuss this in the commentary at all?