Is there any way to make browsers &/or Silverlight application do a GZIP compression of HTTP requests? Don't confuse with GZIP compression of HTTP responses - I know how to set this up on the server side. What I need is to compress requests as well, and protocol allows that - anybody using it? Configuration tips?
As far as I know, gzip is only a part of the HTTP 1.1 standard for responses, not for request data.
The need for compressing requests would be if you post a lot of data, and in that case you can compress your data in the client application (eg. in silverlight which you have tagged your question with), and send it over as a byte array/stream.
Then decompress the data on the receiving end.
You cannot make them unless you are the author of these websites and silverlight applications. There is no standard for a compressed http request. You can setup your server to support it and document it in your api and that is the best you can do.
If your using WCF in the silverlight application you can compress the message with an encoder. But again this is not discoverable and must be documented.