At the end of Season 2 Nathan's hair was pretty long yet the first episode of this Season he has a hair cut like in Season 1. I know that they changed looks for the new Season for some characters like Sylar and Mohinder for example. Still it just seems like mabey inbetween him coming "back" from the dead and him leaving the hospital for the first time. He could've gotten it. Its a MINOR detail I'm just saying.

i like his shorter hairstyle better