From what I understand, Powell personally hates Clark, along with Shwartzkoph, Clark didn't exactley endear himself to the brass, he just happened to be the most liberal guy who kissed up to the right people (clinton). William Cohen essentially relieved Clark from duty for trying to start world war 3 in the Balkans. If Clark were to run, Powell and Shwartzkoph would automatically jump into campaigning against him, out of pure utter hatred for the guy.
Personally, I think when Hillary makes her run in 2008, he will be named VP on the ticket. By raising his persona as much as possible now, and over the next few years, and as long as he stays loyal (he will), he makes a perfect VP pick in 2008.