It seems that this problem cannot be solved easily. You can google and find something like this http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/789c3e1b-e3ae-476f-b37f-d93ef6d0cb7b/ (approach with glyphs). But taking into account the fact that String is an enumerated collection, sometimes you can solve your problem with markup as follows:
<Grid>
<ItemsControl>
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<UniformGrid Rows="1"/>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemsSource>
<System:String>
Hello World
</System:String>
</ItemsControl.ItemsSource>
</ItemsControl>
</Grid>
(, ) . , , UserControl.
, .