This technology hasn't been stagnant at all! The main problem has been that it's such an unusual and incredibly complex material that it has taken fifty years of research just to get a good grasp on the material itself. It wasn't until the late 1990's when the atomic force microscope (AFM) became readily available that some real break-throughs started to occur. The National Science Foundation is still pouring millions of dollars each year into researching the material aspects of nitinol. While there is still a whole world of improvement to be made in understanding the material before mechanical engineers can begin doing a good job of building heat engines with it, we're a whole lot closer. If you read the literature published by the scientists interviewed in this video, you'll see that they really had more questions than answers. My guess is that you'll start seeing more and more companies build something useful out of it.
The most recent development that I'm aware of is an air temperature generator by Kellogg's Research Labs url removed by mod
