Just a reminder: Any overclocking on the hardware will void the warranty, so try it at your own risk

They won't know it. Even though we overclock it until Vista hangs due to over heat, the clock will restore to the factory default after reboot. They can just know that you have installed a software that enables us to overclock (it's written by the manufacturer themselves) but they can't prove that the damage to notebook is due to overclocking (unless it's badly burnt).

Overclock will increase your GPU card's temperature. Most probably it won't damage your graphics card at that instance. However, it will decrease your GPU's lifespan and eventually your notebook's lifespan. Say your notebook's lifespan is 6 years and due to overclock, it causes more damage on the chips and decrease the lifespan to 3 years. At that time, you won't have anymore warranty, and you might need to buy another new laptop.
wah... it seems that 8400GS overclocked is nowhere near the 8600. No use to overclock already... 

i OC my x1400 during gameplay with cpu undervolted, temp is comparable, max is 68C, refer to http://forum.lowyat.net/topic/493624