Michael -- Not exactly true. While adapters that screw into the front of camera lanses and either reduce or enlarge focal lengths have been around for quite awhile, the focal reducers for astronomical uses fit on the back of the telescope and as their name implies, reduce the effective focal length. The same principle is possible for a camera lens.
The way I understand astronomical focal reducers, they mount in front of the final optical group in the eyepiece and lengthen the effective focal length of the eyepiece. In a dSLR, it would have to mount behind all of the other optics. This makes a big difference.
A teleconverter is an afocal device which makes the coverage circle wider (and dimmer). So in effect, what reaches the film or sensor is an expanded crop of the originally captured image.
A focal reducer does the opposite. It takes a larger image circle and reduces it to a smaller one. Unfortunately, most SLR lenses are designed to "throw" an image circle only slightly larger than the sensor. Once you approach the edge of that circle, the optical quality deteriorates quickly and soon gives way to black (hard vignetting). So there's little or nothing left for a hypothetical adapter to "map" on to the sensor.
In theory it would be possible to create such a device for use with FX lenses on DX sensors, or with certain lenses (such as PC lenses) which start with a larger image circle. But it simply doesn't work in the general case.
"There is no real magic in photography, just the sloppy intersection of physics and art." — Kirk Tuck