New theories on who poisoned Brock:
Nobody ever guess what's going to happen correctly, which means Gus didn't do it, Walt didn't do it, and Brock didn't accidentally do it.
THEORY #1:
Tyrus did it.
Why? Because he's been skimming the meth. He altered the weight on one of Walt's batches a week or 2 ago. By poisoning Brock, he hopes to get Jesse upset enough to overlook minor details regarding the weight and too preoccupied to pay attention. Plus, he really really hates Jesse.
It's probably just about the money for him. We already know Mike makes much less money than Walt, and I'd assume Tyrus makes less than Walt. Tyrus probably just wants a bigger piece of the action.
If you pay attention when Jesse tells Gus about the poisoning, it almost looks like Gus shoots Tyrus a quick glance as if he suspects him for some reason.
Theory 2:
Hector did it...or had it done.
It's less plausible, but he could have ordered the hit on Brock. Jesse killed Tuco and then all that was left of Hector's family, and Gus made sure to let Hector know it. Perhaps Hector still has some connections and wanted to return the favor knowing Brock was like family to Jesse. There's no doubt that Hector and his hatred for Gus and Jesse will factor into the remaining episodes, so this isn't as far fetched as you may think.
[This message has been edited by taint98 (edited 10/5/2011 4:55p).]