I wrote the book on mobile GPUs
All GeForce AGP 3.0 GPUs (NV18/28, and NV3x) have a thermal diode, including the GeForceGo (NV18M/28M/31M/34M/36M) mobile GPU series for laptops. I cannot speak to NV41M/43M, but one can safely assume a thermal diode is still included.
However, you won't see the temperature tab on a GeForceGo laptop drivers The laptop OEMs customize the NVIDIA drivers A LOT, because laptop power-saving technology tends to be proprietary (varies from vendor to vendor). Since an OEM manufacturer can save +/- 5 cents per laptop by eliminating such a circuit, many leave it out (however unwisely). Some OEMs hide the thermal sensor reading to prevent the customer from seeing how hot the laptop really runs. YMMV.
I used to drive a Heisenberg, but whenever I'd glance at the speedometer, I'd get lost.