Looks interesting, but a bit of FUD on their pricing page ( http://www.voltdb.com/pricing ):

Open-Source Community Project: "...This edition is unsupported and most likely to be used in non-mission critical applications."

Anybody knows which traditional database did they compare VoltDB against?

Not legally allowed to say due to the anti-benchmarking license the big vendors use. Maybe I can say it was the obvious one?