If Nancy Pelosi said she was considering impeaching Bush, that would mean one of two things.

1) Cheney becomes President. Do we want that? or

2) She impeaches Bush and Cheney and she becomes President. Doesn't that make her look like an opportunist, corrupt creep? Do you really want her on the teevee saying she's going to manipulate things to make herself President?

After thorough investigations, it may turn out that impeachment is the best options. Discussing it now is counterproductive or worse.