I'm very excited to hear that there is a Silverlight control for Bing maps! Here is the link to MSDN documentation. It was just announced at TechEd Europe yesterday!!!! Here is more on that news. It takes less than 15 minutes to get up to speed on using the maps control. I've had some experience creating map control in ASP.Net using Google Maps. The implementation is similar but even easier to use. Google requires that you register using a gmail account and a valud URL. They give you a key for a specific URL, which limits you to registering your development environment localhost:{port}. Bing maps for Silverlight allows you to register to get up to 5 keys and does not limit you to your development URL.
If you are writing anything in Silverlight, you have to check this out! http://team.silverlight.net/announcement/announcing-the-bing-maps-silverlight-control/