But Jesse said on the day Brock was poisoned he'd switched the ricin cig from one pack to another, and I got the impression Jesse wasn't around Brock at all that day until the hospital scene.
Of course, Brock could have ingested some of the ricin the day before and put the vial back in the cigarette, without Jesse's knowledge. According to Walt in one of the Tuco episodes, ricin takes about 24 hours to take effect (but this for some reason wasn't brought up during the Jesse/Walt scene, making me wonder if "ricin" works the same way in Season 4 as it did in Season 2).
Gus probably didn't poison Brock -- he became super-cautious only after Jesse brought up the ricin poisoning; if Gus had done the poisoning, I think he would have left someone to watch his car. And Tyrus sneaking into a house and slipping ricin into some kid's chocolate milk seems outlandish -- Gus takes his cue from Machiavelli, not Boris Badenov.