The following xaml causes the text “Activate a test to the left.” to be visible at run-time and at design-time (in Vs2010):
<TextBlock TextWrapping="Wrap">
<TextBlock.Text>Activate a test to the left.</TextBlock.Text>
</TextBlock>
The following shows nothing at run-time, but the text IS visible at design-time:
<TextBlock TextWrapping="Wrap">
<TextBlock.Text><![CDATA[Activate a test to the left.]]></TextBlock.Text>
</TextBlock>
What’s the problem?
Here is an extensive discussion on this very topic:
http://forums.silverlight.net/forums/t/187623.aspx