Are any kind of wood-chips good enough for smoking? I was watching a Heston Blumenthal Show called "Cooking like Heston" the other day, the episode was on potatoes and he made doughnuts using them. one of the things he did to prepare the potatos for the doughnut mixture was to smoke them using wood-chips. 

I have seen wood-chips be used by him, Jamie Oliver and on Iron Chef (Japan) and Master Chef (Australia) a few times and i've been wondering if any kind of wood-chips can be used to do smoking (ie. ones picked up from the gardening section of a hardware store or made by myself from junk off cuts from trees or building material) or if i should be looking out for a particular kind which is meant for cooking.
You can't just pick up any old wood chips but you can get ones for cooking at the hardware store. Even Home Depot or Lowes should have wood-for-smoking available.

You have to be careful since if you get chips made out of treated lumber for example the resulting smoke could be poisonous. Probably not something you want to eat. You also want to avoid poisonous wood or trees.

Warnings aside, you can use just about any non-poisonous hardwood to make woodchips.  Just make chunks or chips out of nice dry seasoned logs.  Usually, people use wood with interesting favors to add such as fruit tree woods (apple, pear, cherry). Other popular woods are mesquite, hickory, or maple.