Baking AO

I am an experienced Blender user and have baked AO countless times. Today though, I went to bake an AO and I get an error warning “no objects found to bake to”. Materials and Textures have been properly set, UV unwrap set but still the same error.
What am I missing?
Blender 2.67

Do you have Selected to Active checked? If so, do you have only a single model, or one high-poly and one low-poly? If only one, uncheck that. Materials and Textures are set, but do you have a blank image set to be the target?