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What was Walt doing the whole day when no one could get a hold of him? Watch that big black dude searching Jessie again looks more like he's just going into his pockets ... perfect time to make a switch.
Walt had about zero time to take those cigarettes. That night he visited Jesse he was tazered and taken to the desert. He had to make it back home as soon as possible to warn his family, go a little insane, and get them out to Hanks house. By the time they left for hanks Jesse was at work already.
There was no window of time for Walt to get those cigarettes and poison the kid.. He doesnt even know who those people at Jesses house were.
Gus on the other hand, as pointed out in this thread first, and in episode 12, has cameras just about everywhere. He knows who those people in Jesses house were and how important they are to him. Shit he has Jesses BLOOD TYPE just waiting for him in a hospital hes never even visited. Gus has the history of well planned out moves and never failing.
Gus is way too ready for anything NOT to have pulled it off.