How did Walt get the cigarette from Jesse? Even if Walt poisoned the kid, the cigarette should still be in Jesse's pack. Walt had NO opportunity to remove it. In the fight, Jesse kicked his ass and kicked him out. Jesse also said he had transferred it that morning. Walt never saw him that day. Jesse was only showed at the lab, where Walt pointed out that it would have been easy for Tweedle Dee to scoop it out of his clothes in the locker.
It's not that I don't think Walter is capable of it, I just don't think he did it. I did have a thought today, though, that Walter will eventually kill Jesse next season.