The New York Times wrote a great piece on car insurance for modified cars. (Look for the quote from me.) The introduction to the piece sums it up nicely:
Few drivers realize that aftermarket parts are generally not insured, so if you have an accident in your modified car, your insurer is not likely to reimburse you for the damaged add-ons. Even worse, in some cases insurers may deny the whole claim or cancel the policy.
The bottom line: if you have a car with modifications, check it once, check it twice and check it three times to make sure your insurance policy covers you. If you have any questions, ask your agent and get it in writing!