Dick Cheney occupies a historically unique position: He is an ex-VP who left office electorally undefeated and has not sought the Presidency. As a result, he retains some of the trappings of an undefeated elder statesman.
Pelosi's first priority is to maintain her speakership, and that means protecting her Democratic majority through the 2010 elections -- even if it means bucking her own party's president.
Don't buy into the media narrative that this is some sort of "divided party" -- because today's Democrats aren't. All true Democrats are pulling hard to defeat the legacy of George W. Bush.