I got lucky and was out at the right time looking at an Iridium flare to catch this. For the record, here's an attempt to describe how it looked to me:
I was in Temecula (inland between San Diego and Los Angeles) and noticed it first as a contrail with a bright spot at one end and thought - wow, that's a cool fireball/bolide! At the time it was yellowish. It then blossomed into a small glowing ball (staging?), and a few seconds later a much brighter white blossom (second stage ignition?) that created a visible shockwave through the rest of the cloud! It's really hard to describe how unusual and awesome seeing that was! The core seemed to glow bluish, but the white cloud expanded around it, fading very gradually, and at one point covered nearly my entire forward view (i.e. not half the sky, but a very big corner of it). My impression was that the contrail started too far south to be Vandenberg, so my best guess at the time was San Clemente Island. That area would make sense for a sub launch as well.
It looks like the missile must have tracked up the coast, and my perspective was aft quarter to tail-on. The first twitter photos posted up thread are from Santa Cruz and it looks side-on from there with the white cloud ahead (to the right of) the blue portion.
I assume the white cloud was sunlit exhaust at altitude, but what of the blue glow (which persisted for maybe as long as 10 minutes after?) Is it just a different exhaust component being lit by the sun? Or would some rocket exhaust ionize and glow like that at altitude?
I managed to get my camera out and fumble around in the dark to get some mediocre photos (handheld DSLR in the dark, ugh!), but they give a sense of the persistence of the blue cloud - the timestamps on the photos are 18:01, 18:02, and 18:06 (PST) respectively.