Every once in a while, you come to an obstacle that requires you to dismount to traverse it. Hell, if these guys can walk their bikes up a hill, then I think we're excused.
Seriously though, I spin up hills when I can, and walk them when I can't. There's no shame in it. As I get in better shape, I find myself walking less and less - the last two times a hill beat me, it was only because my chain came off.
-DR