Is it possible to use a Silverlight control inside a CRM form? Either versions 4.0 or 2011.
Yes. 2011 allows you to embed silverlight controls via web resources directly in the page. The canonical hello world version of that is here: http://www.dynamicslight.com/2010/10/building-silverlight-web-resources-for-crm-2011part-1/
You can also use Silverlight w/ CRM 4.0 via IFrames, using the URL of the iframe to pass in the necessary GUIDs to your app.
http://crm.davidyack.com/journal/2009/11/18/mixing-silverlight-4-with-dynamics-crm.html