Impeachment is not on the agenda
I agree that Impeachment should not be an overwhelming issue for the Democrats should they come to win the House in a couple of weeks. However, you never say never in politics. Thus, Nancy Pelosi makes a mistake when she says that Impeachment is "off the table". She is right that Republicans would love it since it would play into their talking points that Democrats are defined by Bush hatred. But, again, never say never.
A better phraseology would be, "Impeachment is not on the agenda." This would make it clear that Impeachment is not going to be a goal of the coming Democratic congress. But that oversight and investigation which might inevitably lead to Impeachment can never be ruled out.
Repeat after me: "Impeachment is not on the agenda".