Best way to notify clients for an update?
I think that the pull model that you propose is probably the easiest to implement and most straightforward solution to the problem. Knowing that you are using WCF webservices, however, you have the possibility to implement a push model using WCF Callbacks. In this model you write service contracts just as normal but here the clients register themselves on the server. On data updates on the server the server calls a callback function on all registered clients, which respond by fetching new data.
The typical showcase application for this model is a ticketing system, where tickets are booked and released often. This approach eliminates the need for clients to constantly poll the server.
The article called Eliminate Server Polling with WCF Callbacks gives more details on the subject and also comes with a ticketing system example. This article also describes this method.
I am not saying that this is better than what you suggest yourself, but it may be worth looking into.