Looks like energy sector, or more specifically, oil & gas companies were the primary target. Here's a short synopsis of the attack techniques used, which begin of course, with one of the most common (and easy to defend) attack vectors:
The attacks began with a SQL-injection technique, which compromised external web servers. Common hacking tools were then used to access intranets, giving attackers access to internal servers and desktops. Usernames and passwords were then harvested and after disabling Internet Explorer proxy settings, hackers were able to establish direct communication from infected machines to the Internet.In my experience, oil & gas co's generally have more budget to spend on security protections than their electric utility brethren. So if they don't have their cyber houses in order yet against simple stuff like this, then it's quite likely that the same attacks would have breached electric co's as well.
Click HERE for a short article on this, and HERE for the more detailed report by McAfee.