Well, think of the DNC tv ads that can be run. Ms. Rice sitting before congress, forced to read the subject of the famous ignored memo, "Bin Laden determined to strike US..." etc. She is at the very least a co-dependent to two of the major foreign policy debacles, and as Iraq rages on, she is an easy target. Her being joined at the hip trumps her being black or a woman. When it comes to policy, she's about as white male conservative establishment as it gets.
A bigger problem: She's never been vetted as a candidate. No one knows her opinions on ANYTHING other than Bush's middle east f--k-ups. From gay marriage to SPAMALOT, which she attended after a full day of shoe shopping during Katrina. She's very vulnerable as a candidate, and the Republicans know that any initial "ooooh, a black woman!" angle will be trumped by the many unanswered questions about her.
"I'm a comedian, but in my spare time, things bother me." Garry Shandling