I might say that beer bottles are ubiquitous in the student ghetto where I have the misfortune to live (but it's cheap and I love my little flat! just the neighbours are a drag) - in which case, I would mean that just about anywhere you cast your eye, you'd be likely to see at least one beer bottle within your range of vision. I would never say that beer bottles were omnipresent in the student ghetto: how would anyone move, what would we breathe?!
To me, regardless of definitions (I haven't even looked each word up), ubiquitous means unusually common (if that ain't too oxymoronic!) - something you can expect to see/encounter frequently; omnipresent means all-present. Air is, I think, the only "thing" that is omnipresent on Earth (obviously it's not in space, but), but I think of God as omnipresent. On His bad days, I reckon He probably thinks of people as ubiquitous.
