Microsoft roll out a new performance improvement for the Virtual Earth platform, called “Content Delivery Network (CDN),” which’s composed of multiple geo-distributed data centers (throughout the Americas, Europe and Asia) that allow Virtual Earth to host content closer to customers and end users. As a result of CDN, delivery speeds of Virtual Earth content have been improved up to 82 percent for both the Virtual Earth AJAX Map Control and Virtual Earth Silverlight Map Control. To begin utilizing CDN, you will need to make an opt-in, non-breaking change to your AJAX map control code; no action is required for Silverlight Map Control users, as CDN logic is already built-in. Given the significant performance benefits of CDN, it will be the preferred method of accessing Virtual Earth content.
More info: MSDN