My sincere apologies for the delay in getting back to you on this issue.
Unfortunately Silverlight applications do not allow us to inject click events (as with HTML-based web applications) and so we are forced to simulate system wide mouse moves, clicks, and key presses instead. As you suspected, moving the mouse or sending key strokes require an unlocked desktop, this is a Windows limitation you will find with any tool that performs UI testing.
Please refer to this knowledge base article for additional information which includes a few workaround options certain customers have been successful with in the past.
Let me know whether I can answer any additional questions.
the Telerik team