If you have noticed, the script URL to use for the v5 version of the Virtual Earth API has changed from the format of previous versions. The Previous v3 and v4 versions of the API used the following format:
However, the script URL has changed for v5 going forward to the following new format:
The new map control handler takes a "v" version parameter where you can specify the version of the API. This new handler format is designed to work better with asynchronous frameworks such as ASP.NET Ajax. The previous v3 and v4 mapcontrol.js files would indirectly include an additional script which would make it difficult to asynchronously include the Virtual Earth API. However with the new map control handler, you can include the script portion of the control in one request.