Well, I certainly didn't expect the bomb to be a person. Question is, how Radioactive Man is going to get to New York. Considering how his wife's dead, he's got nothing to lose.
And Micah's power (come on we all knew he had one

). I'm thinking it could be more than just being a technopath and maybe a form electromagnetism, sort of like Magneto. But hey, if his dad is in a jam and needs a quick getaway, his kid is ideal for hotwiring a car.
As for the appearance of the symbol on radioactive guy and Matt's shoulders, I'm thinking it supposed to be a sign that their powers are increasing and developing, a sort of genetic "birthmark." After all, when Niki only has the symbol when she's "Jessica" because, for all intents and purposes, Niki is normal while her other personality is a super. This convinces me that these folks powers were the result of genetic tampering, probably by Mohinder's dad.
I bet the connection between all of the "Heroes" involved some sort of fertility experiment for infertile couples and that eventually, all of the characters with powers either have or are in the process of developing these "birthmarks" the more they use their powers.