I ran into the same problem today when using Visual Studio 2010 with 1 Silverlight Lync project, and 1 ASP.Net Web Application project running out of VS2010 (not IIS7).
When running xap off the file system, there was no problem, however once I ran it in the browser the UI loaded, but failed to load any presence data: "Unknown contact"
Blah blah blah, I was lazy so I uploaded it to Office 365 and again, same problem.
I added my sharepoint (office 365) portal to the trusted sites list in IE and the silverlight app ran without a hitch.
If you can see the UI, but it's empty, issue probably has something to do with security. This is my first day using Silverlight so I really can't say what to look out for other than the trusted sites suggestion.
Hope this helps.