The Environmental Working Group (EWG), a nonprofit working to protect public health and the environment, published their 2011 Shoppers Guide to Pesticides in Produce.
This year apples and celery top the list of items they recommend people purchase organically to reduce the risk of dangerous intakes of pesticides. The EWG measured the pesticides after powerwashing the produce and peeling it as a normal cook might do.
Consider this: a pesticide is a lethal dose of poison meant to kill a living thing. If it kills a bug it may harm you as well. It can be hard to make the leap from spraying for bugs to dangerous to humans - especially since most of us treat our homes for ants and cockroaches. But would you ever spray an insecticide on your hand, rinse your hand with water and then lick your hand? Sounds yucky doesn't it? There would be residue, you would ingest it. Pesticides are made to stay on the fruit even if it rains. They don't wash off with water.
Read the EWG article. It is informative and it gives you a list of produce that pesticide use does not affect, like sweet potatoes and onions, as well as a list of fifteen produce items that you should purchase at your farmers market or organic in the store.
See you at the market!