One problem related to service engine light on has been reported for the 2009 Toyota Prius. The most recently reported issues are listed below.
Service engine light, code p1121, coolant recovery tank flow control valve bad. There is a service campaign on this for 2007 Prius and the problem is supposed to be solve for 2009 Prius. Also this is part of the emission control system, as the purpose of coolant recovery tank is the warm up engine sooner to pass emission test. Called Toyota dealer, not covered by emission warranty. This is a known and very common problem and Toyota is dragging its leg on it.