According to the documentation there, it is defined by the plane equation: ax + by + cz + dw = 0

This is simlar to your definition. To get the exact way to it I think you will really have to do the math. Create three points, apply the transformation matrix them it and see what formula rolls out.

P.s. http://www.geometrictools.com/Docume...mentation.html is a gold mine of information regarding these topics. Alas, they don't handle this one...