G-sensor is a gyroscope, in other words an orientation sensor.
motion sensor is more accurate, it MAY or MAY not have G-sensor's capability, this depends on the software implementation. but the hardware itself is capable of detecting and reacting to all kinds of movement.
G-sensor needs _relatively_ sudden movements to activate the gyroscope (think of a marble in box), if the phones (the box) is in landscape, and you tilt it to portrait, the gryo detects it (ie marble rolls to one side). However, if you keep the phone/box's orientation the same, and move the phone closer or further from you (assumign you ar eholding the phone): now if you move it slowly, the marbles won't roll, this is g-sensor. becaue it can't detect this movement. but if you suddenly jerk the phone/box, even if the orientation is the same, the g-sensor should detect it (ie the marble will move). ummm.. just like those dice games on iphone/itouch (if you 've seen the latest itouch commercial).
motion sensor detects all kinds of movements, it's only up to the software to decide if all functionalities are implented or only "up till" the G-sensor capabilities are implemented or something else. best example, G1's compass view, slow movement with the phone in the same orientation will pan around the map.
that being said even the motion sensor has its limits, if you move the phone uber slowly, i bet it won't detect the motion, unless it's military grade (read: 007 gadget
)