Is there any risk of illness from eating peanut butter?

Yes. The peanut is a common food allergen. Sometimes the obvious bears repeating: Do not eat peanuts or peanut-based products if you are allergic to them.
 
Because of the way they are raised, harvested, and processed, peanuts and peanut-based products may harbor salmonella, a pathogenic bacteria that can result in serious illness. The good news is that peanut butter is heat-pasteurized during processing. The bad: Disease outbreaks involving peanut butter occur anyway. Accordingly, both health and industry officials are examining peanut butter production methods to determine how products involved in outbreaks might have become contaminated, according to the U.S. Centers for Disease Control. One theory posits that salmonella bacteria may demonstrate greater heat resistance in foods with low water and high lipid (fat) content, such as peanut butter or chocolate.
 
Peanut butter doesn't spoil like a perishable food, but it may develop rancidity during storage as a result of its high oil content. Rancidity usually occurs after a year and is characterized by an “old, oily” off flavor. Discard peanut butter if it turns rancid.
 
Source(s):
University of Minnesota Center for Infectious Research and Policy "Peanut Butter Blamed for Salmonella Outbreak"
University of Georgia "Peanut Butter: Check It Out!"
Livescience.com "Can Peanut Butter Go Bad?"