The new Silverlight 3 feature call:
will tell you if your machine as an available network connection.
This allows you to determine if your application is running disconnected from the network (e.g. on a network connection).
One of the purposes of this call is to allow you to decided if you need to store data to isolated storage and sync back to the server later, or if you can sync with the server just now.
However be aware, if you have set your browser to "Working Offline", GetIsNetworkAvailable will not take this into account.