I have recycled some, or rather sold them to be recycled, so i dont know a lot. Heres what i found at ehow.com:

Used X-rays are made of a polyester film coated in emulsion layers made of gelatin and contain silver. All X-ray film contains about 2 percent of its weight in silver after processing. Extracting silver from X-rays can be done in several ways and helps offset Disposal costs.



Anyway they sell for decent money in Michigan.