Completely silent means a laptop that is passively cooled, i.e. no fans. Some netbooks might be that way, but I would trust more a machine that cools itself actively and allows me to run something other than just a browser window at a time. Programs mean CPU activity, meaning heat generated. The only laptops that I would be worried about in terms of noise would be a desktop replacement gaming monster machine. Hint: look at the specs, specifically the CPU and the graphics card. If they are high-end, it will be more likely they will have a bigger cooling solution, meaning greater potential noise.
on the other hand, a redundant cooling solution can offer better cooling for less fan speed, meaning less noise.
The key is in the kind of...