This article reports a study in which the injection of antibodies targeting the beta amyloid plaques into the brains of mice with a close analog of Alzheimer's disease managed to trigger a response in which the immune system cleared the plaques. Neurofibrillary tangles associated with the disease cleared spontaneously shortly after the amyloid plaques vanished. [Update: I've found another somewhat better report from Science Magazine here]
The treatment only worked in mice with early stages of the disease.
This is a very preliminary sort of result, but it is really quite exciting. Even if it does not lead to an immediate Alzheimer's treatment for humans, it does lend extremely strong evidence to the hypothesis that the beta amyloid accumulation in and of itself is the major mechanism triggering the symptoms of Alzheimer's, and that blocking the production of beta amyloid or clearing the plaques would halt the progress of the illness.