Commenting on early Democratic VP speculation, The Garance articulates the conventional wisdom that neither Barack Obama nor Hillary Clinton will put "additional" diversity on their ticket. This, I think, is true, but not necessarily for the reason Garance says. She argues that the ideal Democratic VP candidate is someone from a swing-state who can deliver those extra electoral votes. Wesley Clark, for example, fits the profile because he's "white, Southern, military, and from the potentially swingable (for a Clinton) state of Arkansas" (although Garance thinks worries about Clark's campaign skills may sink him).
Back in 2004, I recalled reading an article about VP selection that argued that so-called "balancing the ticket" is over-rated. It doesn't matter as much to make sure that the south is represented on the Democratic ballot. Rather, what matters is the "narrative" that the newly created ticket presents. What story will the media write about the selection? Selecting a VP is an opportunity to create a positive media theme around your campaign, or diffuse a negative meme. And of course, an unwary candidate can unwittingly construct a very negative image of their campaign. For example, if Hillary Clinton doesn't select someone who can claim some serious military credentials, it will undoubtedly feed into the stock narrative that she's intolerable to (or worse, has written off) the armed forces. (Incidentally, this example shows another truth about media narratives -- they don't have to be true. The idea that Hillary is anti-military is utterly ridiculous, but so long as it's a "perception" it can be written about, and thus reinforced, indefinitely).
Of course, regional balancing cannot entirely be divorced from narrative construction, if for no other reason than that it's been so drummed into us that tickets are supposed to have geographic balance that refusing to do that is a story in of itself. But that doesn't explain why Clinton or Obama couldn't select a female VP from the "appropriate" region. For example, Governors Kathleen Sebelius (KS) and Janet Napolitano (AZ) would seem to fit inside the geographic story just fine (and Napolitano could arguably put Arizona in play). And, to be sure, I've heard both of their names bandied about -- just not for Clinton (or really, Obama).
The reason is that there is a very predictable media narrative that will form if two members of politically underrepresented groups appear on the Democratic ticket. One person is ground-breaking and history-making. Two people, by contrast, is an "affirmative action" choice and proof the Democrats are in thrall to "interest groups." If Obama picks a woman, it will undoubtedly be cast as "appeasing" women's groups who were ready to see Clinton break the ultimate glass ceiling. If Clinton picks a Black running mate, same thing, except replace NOW with the NAACP. This is what Derrick Bell calls the unspoken limit on affirmative action. Even if at first the diversity is applauded, at some point folks will start getting uncomfortable with too many women or people of color. A presidential ticket that doesn't include a White male is virtually inconceivable, and it's equally inconceivable that the media won't make heavy note of that fact in the unlikely instance it comes into being.