For now all polls suggest that the National Front won't obtain the presidency of any region. But polls often tend to understate the National Front.
In the Nord, the Socialist Party has decided to withdraw its list. Thus the National Front is opposed to UMP/Les Republicains Xavier Bertrand. According to polls in this region, 68% of those who voted for the left on the first round will vote on the second. 64% of them will vote for Bertrand and 4% for Marine Le Pen (FN).
Apparently the only region where a FN victory seems possible is PACA, where according to a BVA poll, Christian Estrosi (UMP/Les Republicains) would win with only 51% (49% for Marion-Maréchal Le Pen).
The regional elections are based on a proportional system with a majority bonus: The winner obtains more seats. So even if the National Front doesn't win tomorrow, they will still have an important number of seats.