I think your definition is spot on. The reason you can define a computing device that way and exclude 600 ton airplanes and stuffed toy bears from this discussion is that the devices are embedded in the actual item. An airplane can't (shouldn't) be programmed by the user to perform tasks beyond what was initially designed into the device, but the FPGA's in the teddy bear can be. It's not reasonable to look at how many of today's planes are made by Microsoft, but it is reasonable to look at the embedded systems market as a whole and see how influential Microsoft is.