Silverlight 2 Beta2 includes a bunch of work in the networking space:

Cross Domain Sockets

Beta2 now enables cross domain networking support using both HTTP and Sockets (meaning your application can call sites other than the one the application was downloaded from).

Silverlight will check for the existence of an XML policy file on target servers that indicates whether cross domain network access is allowed.  Silverlight supports a new XML policy file format that we've developed, as well as Flash policy files (which means existing sites open to Flash can be called from Silverlight without any additional work).

Background Thread Networking

Beta2 now allows Silverlight applications to initiate network requests on background threads, as well as process/handle network responses on background threads.  This enables a bunch of powerful scenarios, and allows you to avoid blocking the browser's UI thread while doing both HTTP and Socket network communication.

Duplex Communication (Server Push)

Beta2 enables support for setting up duplex communication channels with a WCF service on a server.  This enables a clean programming model that allows servers to "push" messages to Silverlight clients without the developer having to manually poll servers for changes.  This programming model is very useful in a variety of scenarios, including instant messenger/chat applications, and monitoring/update applications like stock tickers and trader applications.