Some people are mentioning other random possibilities. I think that's missing the point. The mystery isn't really up in the air anymore, with no one suspect able to be especially singled out.
I realize that theories about Little Chicago have been around for years, in every possible flavor. Many of them involved Mab. Hell, some of mine involved Mab. But none of them were really based entirely on fact, because there was nothing to set any one suspect apart from several others. Sure, Mab was über-powerful and interested in Harry, but that wasn't really anything special. Titania is just as powerful, and she might've wanted to save him so that she could take her own sweet time exacting revenge on him later. The Gatekeeper could've done it for his own nebulous reasons, although he would've had to contend with Harry's wards. Nicodemus could've done it using some kind of Fallen super-sneakiness, just to keep a promising recruit alive. Hell, any of the nameless, faceless super-powerful beings in the Dresdenverse could have had their eye on Harry by that point, and any of them might have had their own reasons for keeping him alive.
But in Changes, we found out that, because of her role as godmother, Lea had been following Harry for his entire life, watching over him from the Nevernever side and protecting him from there. Which also means that for the nine months between Dead Beat and the end of Proven Guilty-- and probably for longer, but for those nine months definitely-- Mab was following Harry around, watching over him and protecting him from the Nevernever side.
Anyone could've been interested in him, but it was Mab who was tasked with protecting him, Mab who was watching over him, and Mab who had the opportunity to notice the flaw in Little Chicago's design and decide to do something about it. It was also Mab who would've been able to step right over from Lea's demesne into Harry's laboratory. She might have taken a hit from his threshold, but she's so strong that it wouldn't have incapacitated her.
We have clear, obvious evidence indicating that Mab is the best suspect. Barring future information to the contrary, it's got to be Mab who fixed Little Chicago.