Clark has been supporting Hillary. Given how angry the debate has gotten, I doubt that Obama would tap Clark.
Clark was recently mentioned on Groklaw -- denying ties to the outfit that is proposing a SCO bail out; I thought I saw some mention of Obama, but it looks like I imagined that completely.
Not that I have a better notion at the moment. Edwards would be a good choice, but he has explicitly said he won't do it.
Pique? What about if his "country called him/me"?
Oh, well, looks like all this speculation is moot, anyway:
http://www.theonion.com/content/video/diebold_accidentally_leaks