Today the price of crude has dropped below $90 a barrel. Prices on a lot of other commodities have also dropped. However, it's not likely energy will become so cheap once again that homes and offices will be built with single-pane glass and no insulation like they were fifty years ago. Long-term investment in green tech or green tech products is not going to stop because oil's gotten suddenly cheaper. The payback on energy-saving materials or software is steady and often has an ROI that is essentially permanent.
The reason the market-watchers give for cheaper crude: fear of an American economic recession means fear that the demand for oil will drop. And today there was a move by the Federal Reserve to make money chesper to borrow, so that fueled oil price drops as well. Even the Fed seems to fear the R-word.