A coworker of mine has posted an issue on Microsoft’s Connect site, which is a gateway to presenting MS software issues directly to the developers of MS products, regarding the Silverlight web development platform.
Apparently, in Silverlight, unless text is contained explicitly inside of a TextBox or similar control, you can not use your mouse to highlight and then cut/copy the text for pasting elsewhere. This is pretty standard behavior for text displayed on any other web page or software application, unless the text is rasterized as an image or displayed as a label in a Windows Forms application.
If you have any interest in Silverlight’s growth and you have access to Connect, please go to this issue and validate it. MS has replied that they will correct this only if there is significant feedback from the community. If you don’t have an account with Connect, apply for one, or find another channel to air your feelings on the matter. I can’t believe that MS doesn’t see this tiny bit of functionality as imperative.