The military's approach to that problem is to use color-coded cables and ports, and we already use things like 802.1x to prevent unauthorized devices from being attached to corporate networks. This is not a novel or unsolvable problem.
Again, STUXNET shows that bugs get into authorized devices really easily. One of the biggest issues is you have power guys that don't know dick about computer security using fleets of laptops that will need a huge amount of training to bring them up to the needed high levels of security.
More so there are thousands of these devices out in the field in power company trucks. All it takes is someone picking one of these up and infecting it for the problems to spread.
It is an insanely hard problem to solve. Breaches happen in military networks pretty often. As a measure of secrecy, they never tell you about it.
While you certainly make a good point about the difficulties that come with securing the laptops and other equipment, the military has the same problem. Most of the users of classified military systems are not themselves experts in computer security. The necessary training is not a graduate level course in security; it is training people to always match cable colors to port colors, to only use laptops provisioned by the organization, etc. The military disables, sometimes by physical means, USB ports on laptops that are used on classified networks to close one of the key security holes Stuxnet took advantage of.
Sure, it is challenging, but this is not impossible to solve. Utilities would have to be forced to do it and they would need time to overhaul their operations, but they certainly can do it, and there is an argument that the NSA's IA division should help (as the electrical grid is a critical for national security). It can be done, we just have to be willing to do it (and pay the cost of doing it).