I'm sure others have seen this as well and considered its ramifications, but I thought I'd throw it out there since the topic is presently being discussed. I was watching the Bonus Materials disc of the trilogy DVD set out of boredom and noticed an interactive family tree in which you can select certain members of the Corleone family and read a one or two paragraph character synapsis. With regards to Fredo it mentioned he was murdered on the orders of his brother for providing Johnny Ola's men info about the layout of the compound.

I know it's widely accepted that Fredo opened the drapes, but what if all he did was provide them info about the compound, such as where Michael's room was at. That certainly would seem to be enough info to get the job done. After all, they had managed to elude or kill Rocco's security well enough to be stationed right outside Michael's bedroom window. They were obviously close to the window considering they were using fully automatic machine guns, guns which aren't made for shooting over great distances. Suppose the plan was simply to start blasting once the lights went out. I know they probably would have killed Kay, and maybe even one of the children, in the process, but I highly doubt Ola would have cared. The whole nobility of the Mafia for not killing innocents is proven to be a load of bullshit several times in the movie itself, i.e. Ciccio's men coming to kill an innocent 12-year old Vito, gunning down Tattaglia's whore along with him, or killing the Pope.

I see the reasoning for believing that Fredo opened the drapes, and it still remains a possibility. I don't mean to sound like my theory is absolutely correct and all others are hogwash, espeically since we have no way of knowing. I just think it is far too great a risk for Fredo to have gone into Michael's room to open the drapes. Michael didn't anticipate Fredo's betrayal, but he obviously had suspicions it might one day happen. As soon as the hit was attempted, Fredo was one of the people he immediately suspected could be the traitor. The seed of suspicion must have been planted before that night. With this in mind, I don't think there's anyway Michael would have given Fredo free reign around the compound. Just judging from the controlling nature of Michael's character, I highly doubt anyone other than himself, Kay, Anthony, Mary, and maybe Mama Corleone and Tom ever had access to his room where his wife sleeps and his children play with their toys. I think it's unlikely Fredo had access into Michael's bedroom and even more unlikely he was able to sneak past the men guarding the compound. I think it's much more likely Ola's men were able to silently kill the guards outside the window in the dark, which they obviously did, then it was for Fredo to be able to sneak past the guards in the well-lit, confined spaces of the compound.

Finally, I think you have to consider the source. There are some fantastic minds with regards to the movie, but if the movie makers claim Fredo leaked info about the compound and that was his betrayal, then it's kind of hard to argue with their opinion. I suppose the possibility exists that Fredo leaked the info AND opened the drapes as well, but if the movie makers think it important enough to reveal the actions of Fredo's betrayal, why would they only tell us half the story? Let the debate begin.