I remember seeing articles about a military multifuel motorcycle at least 10 years ago, maybe even in the late 90s. These converted Kawasakis do sound awesome though. I've owned 4 Kawasakis (and written off 3 of them) and I currently own a Ninja. Great bikes.
Edit: There seem to be a few different diesel motorcycles around now. And HDT has an M2 version as well:
http://en.wikipedia.org/wiki/Diesel_motorcycle