Keep in mind a few things:
1. The writers are capable of making a show go pretty much in any direction they want
2. When Glen dropped into the crowd, he dropped back first holding an already dead Nicholas on top of him
3. It's possible we are seeing the zombies eating only Nicholas' guts
4. Michone was able to walk amongst the zombies when she was covered in zombie blood. Glen *might* be able to escape doing the same.
5. According to "Talking Dead", the producers said that Glen would be in subsequent episodes in one form or another as part of the show's continuity.
That's said, the writers could still kill off Glen if they want.