Been wondering that since Day One. In my best dreams, it's been a Rovian Plan with Novak kicking it off.
More likely somebody's brain fart that has brought on a completely unneccesary opening for the Left to attack. Scandal-wise, Bush has done a remarkable job (by recent standards). Alas, there are far too many egomaniaces running around the halls of power for anyone to completely control.
IMO, if it turns out that Libby flapped his lips, he should be thrown under the bus without ceremony. Same for anybody else. If it drags out, the Dims have fodder for their 2006 campaign ("Republicans are corrupt, we are the saviors, etc., ad abusem"). If the issue is closed out fast, the Dims will be forced to do what they have been avoiding for many years -- declare their agenda.