Yes God is omnipresent. Yes hell is the absence of God.

Well, based on what the Bible says, God's presence is in hell. As David says, "If I make my bed in hell, You are there." (Or something a lot like that.) The point is that those in hell have desired by their choice that they want nothing to do with God, and therefore God gives it to them by blinding them from His presence. Hell is the absence of God in those souls' perspective. We can be sure He's there, and we can be sure those in hell can't tell. But this is just one theory...

Kind of like what Chris said....I would relate this to the whole genre of art and the body, where pieces show evidence of a body once being present, but are absent now, and some times they end up being more powerful than the pieces with a body in them. Maybe all those people who denied God's presence are shown how wrong they were when his presence really isn't there.

bravo to harrelson - i agree with his explaination

What Harrelson said. Basically you ask for it and you get it.

When God couldn't look at Jesus on the cross Jesus felt forsaken. Maybe God will turn away from those in Hell too. It's worse to be in the presence of someone who is intentionally ignoring you than not have them around at all. Maybe that'll be part of the punishment.