Microsoft has released a beta of a new portable library called Bcl.Compression that adds support for zip archives and compress streams (i.e. deflate and gzip) for the Portable Http Client. Unfortunately it requires a native library so Silverlight and Windows Phone 7.x developers are out of luck.
The ability to use file compression like the venerable ZIP format is very important to many developers. For those developers using.NET, that means dropping to command shell or using a third-party component. With .NET 3.0, there is built-in support for ZIP files, though the implementation is somewhat questionable.