I was on the United Express Regional next in line behind the Delta regional jet and on the correct side of the aircraft to watch the air traffic. Things were busy on that day (busier than usual at O'Hare) and the runway we were about to use (9L) was also being used for landings. The A319-ER taxiing around in the distance had a livery that I did not recognize (I later found out it was Privatair flying for Lufthansa from ORD to Dusseldorf an all business class service) so I was watching that aircraft closely.
I was in a near panic when I notice the A319 and the Delta regional both begin their takeoff roll almost simultaneously. Now, if I could see the A319, the Delta crew should have as well and agree that it probably should have been noticed sooner. Anyway, lots of speed brakes, thrust reversers and tire smoke later they were both in contol and I would dispute the 100 ft separation as being at least triple that distance. Neat nosegear contraction on the A-319.
My belief is that the Delta crew reacted before the Privatair.....wish I was listening on the tower frequency during this one.
The aircraft then taxiied to a new position, (a little slower than usual; maybe they were a bit shaken) with the Privatair A319 now a couple of aircraft behind the aircraft I was aboard.
Arriving in Atlanta, I asked my pilot his take on the "incident" and he commented it was close (perhaps too close) and his opinion at that time was controller error.