DeletedUser
Yes, but that hardly means it's OK to use. Unless specifically stated, copyright artwork requires the copyright holders written permission to use said artwork in any way, commercial or non-commerical. It's the intellectual property of the copyright holder, to be used how they want it used.
I'd like to see the face of Bungie managers if creators of all the Halo sigs out there would mail them asking for a written permission. And I personally got Halo game just because of the sheer numbers of its use in sigs all over the net ("if it's that popular, must be any good").
It all comes down to common sense, really. You're quite OK to use something if your actions don't harm other people interests. Of course, copyright holders still have the right to ask to remove the work if they think it's not in their interests. It's not a big deal to take down a sig or two if you're nicely asked to do so.