To Barnett Rubin, it might be a first shot in a bigger war implicating Pakistan and Afghanistan both. For my part, I think a lot of things have to go wrong to reach that point, and I don't think we've lost Afghanistan...yet. There's still time, but only if we redirect our forces and energy from Iraq.
UPDATE: Via Juan Cole, we learn that leaders in Bhutto's Pakistan People's Party say they were warned of a possible attackin the days leading up to Bhutto's return to Pakistan, and this NY Times article notes that the street lights failed as the sun set, something that would of course make it difficult to spot a suicide bomber trying to get close to the convoy. Evidence of a wider conspiracy? We'll see.