Fortunately, no one was injured. I was slowing down in traffic (to around 10 mph) and the guy behind me didn't immediately notice. His slowing, though audibly sincere, was a bit delayed; I'd guess he was going around 25-30 mph when our bumpers connected. I realized that a collision was imminent too late to really do anything other than face forward. Quite a thump in the back, let me tell you.
No injuries, just bumper damage on each car. He offered apologies and it looked like we had about the same reaction: Damn, this sucks. Well, it can be fixed.
I exchanged information. I also took some photos of both cars at the scene (I-880S near Industrial). Keeping a disposable camera in the glove box was a great piece of advice given to me by patch_bunny.
So... Later today I get to go visit the insurance office and get a damage assesment. Then talk to the garage about fixing it. Fun fun. :P