In the comics Glenn dies another way. By a person that hasn't made an appearance on the show yet. Although the show and comic are not following each other.
My bet is he comes back alive and dies at the end of this season another way.
I don't think he's dead. I believe the walkers were eating Nicholas' innards. The way they were ripping him open, there's no way he'd have been alive THAT long while he was being disemboweled.
Also he wasn't featured in the In Memoriam section of The Talking Dead. They've faked out people before (ex. Baby Judith) and we were led to believe they were dead, but never got the memorial on TTD, and they ended up being alive. Until Chris Hardwick announces it, they should be assumed alive, imo.