Quite a lot of people don't think prices can fall that much further as that would mean prices below the marginal cost curve - but (a) prices can fall below this no problem (at least in the short term), and ( the marginal barrel of oil changes as demand contracts (i.e. it stops being that expensive to refine Canadian tar sands and becomes more easily extractable Middle Eastern oil (hence the marginal cost falls), plus © in a global recession costs (such as engineering and steel costs) also fall, further diminishing the marginal cost.
...so yes prices can certainly fall quite a bit further, $30 by end 2009 is my call