Im not sure if thats GDK.Net or a class library u r using with C++ GDK, but try this :
dbRotateCamera(0, (dbCameraAngleX()+dbMouseMoveY()), (dbCameraAngleY()+dbMouseMoveX()), dbCameraAngleZ());
instead of sending the mouse position to each of the X Y Z co ords in the rotate camera function...
send to X - Camera X Angle + Mouse Move Y
send to Y - Camera Y Angle + Mouse Move X
send to Z - Camera Z Angle
you can also apply different modifiers in the equation to get different speeds of rotation.
If it ain't broke.... DONT FIX IT !!!