O.o
To answer the question no, some manufacturers like Dell do not void your warranty if you change the thermal paste. I have changed it and had a dell technician come out to replace the fan and had no problems. If you go to other forums like notebookreview they also say that it is ok to replace. Some people have even broken stuff and it was still replaced by dell but I would be careful as that may not happen. But as long as you look it up and do it correctly there should be no problems.
As for the comment that it makes no difference in a laptop, that is rubbish. I suggest you google the difference thermal past makes any where from 5C to 15C. I have an geforce 260M GTX over clocked from 500/900/1350 to 700/1100/1750 running at full GPU load at 66C. I do have an extra laptop cooler under the laptop. But i can tell you without the fans it is still 10C cooler and this makes your graphics card last longer and the fans are quieter.
Google how to nvflash your nvidia gfx card and google how to take your laptop apart and apply thermal paste. I have done it to all my laptops.