Ok, I'm assuming you're using a perspective projection here...
Using the parameters you used in glFrustum/gluPerspective, you should easily be able to find the location of what m was before applying the MODELVIEW matrix. R is essentially the near clip plane, I think. (If I'm understanding correctly) So the z is easy. x and y would be found by knowing what your left, right, top, and bottom clip planes are. (If you use gluPerspective you can calculate those using the FOV, aspect and near clip plane. Think of those as defining a pyramid, then you should be able to figure out the trig involved.)
Once you have that location, just multiply it by the MODELVIEW matrix after any "camera transformations" you may have done.
[This message has been edited by Deiussum (edited 05-30-2003).]