There was a bill making its way through both houses of the Diet that would essentially strip the utilities of the monopoly they have over power generation and distribution, which was approved by a lower house committee a few weeks ago, hence the drop to below 600 yen. However, reports were saying that the bill was "expected" to pass both houses (thus becoming law) before the conclusion of the current Diet session, when ends on June 26. I have yet to see any reports so far saying that the bill was passed by both Houses.
The LDP (current in-power party and friend of Japanese Big Business everywhere) doesn't want to appear beholden to the power industry, but at the same time they realize that breaking up the monopoly of power generation and distribution will cause a disruption to the "stable power supply" that Japanese business leaders are so obsessed with, and most likely cause electricity prices to rise even further in the short-term, thus hampering the economic recovery, even if it creates more "competition" in the long term. Lucky for us, elected officials are almost always more concerned with the short term. Given that the general election for the upper-house is on July 21, and the fact that the LDP is polling at above 40%, with its closest rivals, the DPJ pulling under 10%, my guess is that the bill doesn't get put up for a vote until after the election, when the LDP has a super-majority control of the entire Diet. Assuming that, I expect the bill to be voted down, giving us a nice bump in the share price.
Of course, I could always be wrong, and the LDP might decide that passing the bill today/tonight will solidify their victory in July. But with the huge discrepancy between the two parties (40% vs. 10%), I think the LDP realizes they have the election in the bag, especially as they've been distancing themselves from the more radical members of their party recently, like the Mayor of Osaka. Either way, I wouldn't expect much price action until after July 21.
As with all my investments, I always try to assume that politics will go against the company (short of bankruptcy) rather than for and then assess whether it's still worth investing. As far as I'm concerned, regardless of whether they are going to break up the monopoly, which to me is already a given, it's not going to have a huge impact on the share prices of the respective utilities. I think that it has already been priced into the share price.
That said, it looks like both of us agree that any real price action will be in July...