1. A document annotation system comprising: a graphical user interface for annotating documents, the graphical user interface including at least one user input device and a display device configured to display documents; a probabilistic active learning component configured to train an annotation model and to propose annotations to documents based on the annotation model, the probabilistic active learning component also outputting a probability of acceptance associated with each proposed annotation; and a request handler configured to convey annotation requests from the graphical user interface to the active learning component and to convey proposed annotations from the active learning component to the graphical user interface, the request handler including a mode selector that selects at least between (i) a training mode in which low probability proposed annotations are presented by the graphical user interface and (ii) an annotation mode in which high probability proposed annotations are presented by the graphical user interface.