Starting on April 11, 2012 we will require the API version string to be included as part of the HTTP request when downloading the AdWords API AdHoc reports. If version is not provided, the RequestError.INVALID_INPUT error will be returned. The requests for downloading reports should be sent to the following URL,
If you are using the latest versions of our client libraries for downloading reports, this change will not affect you.

As always, please feel free to ask any questions regarding the AdWords API on our forum or during scheduled office hours.