If God is omnipresent, is he also in hell? Using the 'separation from God' concept of hell - how can God be in all places yet still place a soul into a chasm of separation.
Yes, God is also in hell. The separation is not from his presence as if there was a place where his power was in effect, it's a severing of the relationship with him. The thing that will be experienced in hell is the wrath of God against sin rather than the grace of God towards his Son.

I have heard this quip from several preachers so I don't know who originated it, but there is a poignant quote about heaven and hell:


  Heaven is heaven because God is there. Hell is hell because God is there.


We see over and over again that Heaven is defined by the presence of God, his holy purifying presence. Unfortunately hell will also see some aspects of his presence as he stands in utter severity against those who would defy his holiness. Those there will be separated from his grace and any relationship with him other than being objects of wrath.