Using Google App Engine as Proxy for Silverlight Cross-Domain Requests

When using Silverlight or Flash to fetch data from other domains one often runs into cross-domain access restrictions. For security reasons in order to access data from different domains, the remote server explicitly has to allow this by defining a crossdomain.xml (or, for Silverlight, clientaccesspolicy.xml is good as well). If this file cannot be found on the remote domain, the request is not executed. In this article I'll be showing how to use Google App Engine to create a simple proxy that will forward these requests for free - within a reasonable daily load.