We’ve had computer worms before, but Stuxnet is the first I’ve heard of that was written to deliberately target something other than just another computer – that is, to affect the real world.
There are good discussions here, here, here, here, here, and here that are worth reading.
I certainly don’t know who wrote it or why, nor do I have the geopolitical knowledge to forecast many of the consequences that can be expected. It is an escalation of the state of conflict between computer security and system attackers.
It appears that the people behind Stuxnet are well-funded and very capable. One wonders if they have any other development efforts in progress, or if there are other similar groups at work already.