BS. Unless Obama magically stops being half-black sometime in the near future, he fears a primary challenge like bear fears a bunny.
So long as Obama enjoys near 100% support from black Dems - and that is NOT going to change - no one can unseat him. Democrats (and especially the far left) made their racial-identity political bed, now that have to sleep with it.
A Democratic Party that ditched the incumbent, first-ever black President in favor of some white alternative would be cutting its own throat with its single largest voting bloc. Some marginal alternative candidate may run, but the Democratic Party establishment must back Obama. Better for them to lose in 2012 and live to fight another day than to alienate black voters.
For better or worse, they're stuck with the Zero in 2012.