From their FAQ:
http://www.wd40.com/Brands/wd40_faqs.html
What surfaces or materials are OK to use WD-40 on?
WD-40 can be used on just about everything. It is safe for metal, rubber, wood and plastic. WD-40 can be applied to painted metal surfaces without harming the paint. Polycarbonate and clear polystyrene plastic are among the few surfaces on which to avoid using a petroleum-based product like WD-40.