This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
On a very fundamental level, that will require the "master" map to communicate any noteworthy changes back to the server (most likely using an AJAX library like Prototype or jQuery), then a server component that accepts these changes (maybe a servlet), and finally all other map instances must check with the server every so often if there's anything new, also using AJAX or maybe Comet.
Ping & DNS - updated with new look and Ping home screen widget