A word of caution

Different 3rd party service providers have different rules on using their APIs (including feed URLs) for fetching content. Some have restrictions on the number of requests a client (i.e. the public IP of your service) can request per hour or day. Others have restrictions on using their services for commercial use. For more information, please refer to the API documentation of the specific service you want to make available to your site's users.

If you are experiencing problems with some 3rd party services, the solution may be to develop a proxy server that VCE relates to. This proxy deals with distributing the requests among several public IPs or provides API key information for the third party service. This is more or less the way Facebook and iGoogle application works. These sites cloak (using IFRAMEs) the app which is hosted on a dedicated server entirely. The only thing that makes it blend in with the mother web site (Facebook and iGoogle) is the skinning of the contents (and often the lack of IFRAME scrollbars). Depending on the 3rd party apps you want to make available on your site, you may or may not need to develop a proxy service for (some of) them.

You will find that many services will work effortlessly without any additional proxy layer, whereas others will pose problems as the user base increases. Reading through the 3rd party service API documentation will help you in making the right measures to ensure seamless integration.