Here's where the logic fails for me:
When mammacita mentioned the flu-like symptoms that didn't get better, I automatically thought of Jessie's ricin. Jessie made the connection after seeing his pack of smokes. I then thought that the kid was the dumbest thief in the world. If you are going to cage a cigarette, you don't take the one that stands out, the one the owner of the pack will know is missing. You sneak one of the unassuming, ordinary smokes.
Apparently Jessie never thought that the kid could have been wholly responsible for his predicament. Jessie lept to the conclusion that baby tio was poisoned, automatically blaming his erstwhile partner. As Walt correctly pointed out, the timeline to make Walt responsible for the poisoning makes zero sense. Walt didn't have access to Jessie's cigarettes (the fingle fingers of Saul not-withstanding), the kid and the kid's mother knew Walt's face and would have been wary of accepting anything from him and Walt has more time-sensitive issues than a spite poisoning.
So, the conclusion that Gus was behind the poisoning was quickly reached. Walt stated that Gus must have done it to drive Jessie to rage so hard he offed Walt. But, the mystery was unraveled so quickly, that it stretches my belief that a man as calculating as Gus could have relied on so flimsy a plan. Therefore, I conclude that Gus wants Jessie to know that he poisoned the kid and wants the poisoning to act as a warning of some sort to Jessie. However, we have seen Jessie's nihilistic streak and a dude as smart as Gus should know that killing this kid is the absolute worst way to coerce cooperation out of Jessie.
In the Gus-Jessie interaction, Gus made no sign of tipping Jessie off to the truth. If Gus is relying on this event to act as a way to force Jessie away from Walt, he did an awfully poor job of planning. If Gus did this as a subtle warning to Jessie, he completely misread Jessie's character. Both options are completely out of place from the calculating chickenman we have grown to know and love.
In my imagination, once the kid was in the hospital, I thought the story line would prove the kid caged a smoke and managed to unintentionally kill himself. This death would send Jessie into an even more nihilistic state than after the death of Jane. Walt and Gus would have to form an uneasy alliance for Gus would need a cook and Walt would need protection from a former protege now set on revenge against Walt, Gus and anyone else that Jessie associated with on his meth journey. The first victim of Jessie's vigilante revenge would have been poor Badger. Walt's "heinous" action would have been killing Jessie before Jessie managed to complete his revenge vision. Even better is if Jessie manages to take out Gus first, allowing the ascension of Heisenberg and setting the stage for a Season 5 Hank-Walt showdown.
Now, I just have to trust the writers to be better than my inane ideas, a feat that is not hard at all.