Perhaps to prepare for Leopard’s Quick Look and Cover Flow view, Garageband ’08 now saves a screenshot a project’s Arrange window.
It’s all in the .band file: in the contextual menu select “Show Package Contents”. The preview is held in the Output folder and is updated when the project is saved.
I don’t believe that earlier versions of Garageband have done this, but I can only compare it to the first version. Since Cover Flow and Quick Look are notable Leopard features, it’s likely that the next version of Logic will do the same thing.