(Portland, OR)  --  Police say they've arrested a driver accused of hitting a bicyclist with his Mercedes convertible and then leaving the scene after giving his victim "the finger" near Forest Grove.  The Washington County Sheriff's office says deputies arrested 55-year-old William Offinga yesterday after receiving several tips from the public.  Police say a bicyclist was on the shoulder of the road looking at a map near Northwest Hillside and Clapshaw Hill roads Saturday afternoon when Offinga crashed into him and then gave him the finger as he drove off.  The bicyclist survived with injuries.