I have a silverlight 2 beta 2 application that accesses a WCF web service. Because of this, it currently can only use basicHttp binding. The webservice will return fairly
WS-Compression for WCF allows you to configure compression on the binding.
See WS-Compression for WCF by Pablo M. Cibraro
Alternatively, try Microsofts GZip Encoder Sample which "creates an encoder channel that uses the System.IO.Compression.GZipStream class to compress outgoing WCF messages"