Was really surprised to see that Dorgan won’t be seeking reelection this year but I’m not buying his reason:

In his statement, Dorgan said his retirement was borne out of the desire to spend more time with his family.
“Over this holiday season, I have come to the conclusion, with the support of my family, that I will not be seeking another term in the U.S. Senate in 2010. It is a hard decision to make after thirty years in the Congress, but I believe it is the right time for me to pursue these other interests,” Dorgan said in a statement.
“Let me be clear that this decision does not relate to any dissatisfaction that I have about serving in the Senate. Yes, I wish there was less rancor and more bipartisanship in the U.S. Senate these days. But still, it is a great privilege to serve and I have the utmost respect for all of the men and women with whom I serve.”

I think this is more likely to be the reason:

A December Rasmussen poll showed Hoeven leading Dorgan by an astounding 22 points, 58 percent to 36 percent. Both candidates, according to the poll, held very strong approval ratings within the state.