After the fiasco of the mass poisoning scene, it is possible that the writers mean to reveal Walt as Brock's poisoner. It would be a bad plot development because, despite claims to the contrary, Saul and Saul's guys would not be on board with the plot, save by decree of script.
Further, they could not easily poison Brock even if they wanted to. Children are not generally left lying about for strangers to put food, drink or medicines into their mouths. Indeed, any poisoners would have to know the family schedule to even have a hope. (I have it! Jesse poisoned Brock!)
Walt could not possibly know he could talk Jesse around before Jesse blows him away. He knows perfectly well that Jesse fundamentally hates Fring as an evil man, something that Walt does not. Walt merely fears Gus and envies his power. If Walt had not been so grossly, mindlessly abusive, Jesse would never have been even tempted.
There's also a problem with Fring's motivation. He must know, at some level, that Jesse is his personal enemy in a way that Walt simply is not. There's no reason Fring couldn't have just kept Walt as a highly paid specialist employee, while playing a waiting game. If the cancer comes back, he finds another Gail. But to rely on Jesse? Jesse was good for throwing to the Mexicans. He never would have been a happy camper in the organization.
Incidentally, Zap2it did a slide show on anti-villains, but omitted Jesse Pinkman, one of the best current examples.
