Consumer Reports has a great, short article, with some solid science behind it, about when it pays to buy organic and when you shouldn't bother.
The short list:
ALWAYS buy organic if possible: apples, bell peppers, celery, cherries, imported grapes, nectarines, peaches, pears, potatoes, red raspberries, spinach, and strawberries (reduced pesticide exposure); meat, poultry, eggs and dairy (avoidance of hormone and antibiotic supplements; reduced likelihood of prion exposure); baby food.
Buy organic IF MONEY IS NO OBJECT: asparagus, avocados, bananas, broccoli, cauliflower, sweet corn, kiwi, mangos, onions, papaya, pineapples, and sweet peas; breads, oils, potato chips, pasta, cereals, and other packaged foods, such as canned or dried fruit and vegetables.
Don't bother: seafood, cosmetics.
ConsumerReports.org - Organic products, when buying organic pays (and doesn't) 2/06
When the going gets weird, the weird turn pro. - Hunter S. Thompson