Picking up stuff from 3D-space with mouse rays

Okay so I finally have it figured out thanks to the nice people at microsoft.public.win32.programmer.directx.managed newsgroup.

I'll just post the code here too in case someone else might someday run into a similar problem, and to be honest, there are no good examples of doing this unless you're using a Mesh object.

//Checks if point P is inside the plane
internal bool PointIntersects(Point p, Device d, out IntersectInformation intersectInfo)
{
            //Create near and far vectors from the Point location
            Vector3 near = new Vector3(p.X, p.Y, 0);
            Vector3 far = new Vector3(p.X, p.Y, 1);

            //Unproject the vectors into world coordinates
            near.Unproject(d.Viewport, 
                           d.Transform.Projection, 
                           d.Transform.View, Matrix.Translation(location));
            far.Unproject(d.Viewport, 
                          d.Transform.Projection, 
                          d.Transform.View, Matrix.Translation(location));

            //Calculate the direction of the ray from the vectors
            Vector3 dir = Vector3.Subtract(far, near);

            //Set up the locations of the vertices 
            //in the first triangle in the quad
            Vector3 loc1 = location;    //Bottom left
            Vector3 loc2 = new Vector3(location.X, 
                      location.Y + size.Height, location.Z); //top left
            Vector3 loc3 = new Vector3(location.X+size.Width, 
                      location.Y, location.Z);    //bottom right

                        
            //check for intersection
            bool intersection = Geometry.IntersectTri(loc1, loc2, loc3, 
                      near, dir, out intersectInfo);

            //If we didn't find an intersection in the first triangle, 
            //check the second
            if (!intersection)
            {
                //Set loc1 to the coordinate of the top right vertex
                loc1 = new Vector3(location.X + size.Width, 
                           location.Y + size.Height, location.Z);

                //Check the other triangle                
                intersection = Geometry.IntersectTri(loc2, loc1, loc3, 
                           near, dir, out intersectInfo);
            }

            return intersection;
}

I used that inside my Plane class which is used for rendering 2D-like quads.