The elearning Guild's Learning Solutions e-magazine recently published an article by Brandon Carson, Dolly Joseph, and Enzo Silva tilted "ARGs Leverage Intelligence: Improving Performance through Collaborative Play."
It is a worthwhile read. It includes some great examples of ARGs and a case study of an ARG implemented by Sun Learning Services (Sun Microsystems). Plus, there is practical advice regarding designing ARGs for learning, who should be on an ARG design team and their roles, and design risks.
FYI: You will need to log-in to the e-Learning Guild to access the e-magazine. If you are not a member, you can join as an Associate Member for free.
Here are few more resources on ARGs and learning:
Alternate Reality at the Smithsonian
Innovative Learning (including examples and design principles)
ARGology.org - ARGs in Education & Training (contains resources for getting started and examples)
Series of video interviews regarding games and learning from Frontline (PBS)