Stuxnet accidentally released from research laboratory
If Stuxnet had been coded by a nation state you'd be seeing scenes that you see in Japan right now with cooling systems failing and meltdowns imminent.
Stuxnet was a sloppy job for any nation state standard, what I think is the virus accidentially leaked from a research laboratory during development.
What I think is there is a bigger Stuxnet that was being developed as a contingency stock pile of electronic weapons, it wasn't being developed for anytime soon, it was simply there as a contingency and accidentially leaked.
I don't think anyone such as anonymous who are reported to have a copy can recreate Stuxnet and develop it as humint and specialised knowledge of key systems are needed to penetrate deep into the energy sector.
What systems Stuxnet was able to penetrate will be of a completely different security setup than they were to disallow a second attempt at Stuxnet.
To recreate a new Stuxnet worm would need newly found knowledge of the places they wished to penetrate.
Unless a bunch of folks get jobs in the energy sector or befriend some nuclear experts, you are unlikely to see another Stuxnet.
The energy sector is on a raised alert since Stuxnet now that this proof of concept surfaced.