I do love Silverlight, but it does seem that the simplest of tasks are often the hardest to do. Today I wanted to have a nice little grid structure as my List Item data template and for one of the grids to contain a text block. There might be lot of text in there so I wanted the text block to be scrollable, sounds easy. So I thought I would just do;
<TextBlock Text="bla…" ScrollViewer.VerticalScrollBarVisibility="Visible" TextWrapping="Wrap"></TextBlock>
Alas that doesn’t actually create a scrollbar – looks so promising. Anyway without boring you with all the various attempts I finally settled on this;
<Grid Height="149" Width="290">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="60"/>
<ColumnDefinition Width="200"/>
</Grid.ColumnDefinitions>
<TextBlock Grid.Column="0" Text="Col0"></TextBlock>
<StackPanel Grid.Column="1">
<TextBlock Text="Header"></TextBlock>
<ContentControl Height="100" >
<ScrollViewer VerticalScrollBarVisibility="Visible">
<TextBlock TextWrapping="Wrap"
Text="Vestibulum vestibulum adipiscing parturient vestibulum vestibulum adipiscing parturient vestibulum
vestibulum adipiscing parturient vestibulum vestibulum adipiscing parturient vestibulum vestibulum adipiscing
parturient vestibulum vestibulum adipiscing parturient vestibulum vestibulum adipiscing parturient vestibulum
vestibulum adipiscing parturient vestibulum vestibulum adipiscing parturient vestibulum vestibulum adipiscing
parturient vestibulum vestibulum adipiscing parturient vestibulum xxxxxxxxxx xxxxxxxxxxxxxxxxxx xxxxxxxxxxx xxxxxxxxxx
yyyyyyyyyyy yyyyyyyyyyyyyyyyy yyyyyyyyyyyyyyyy yyyyyyyyyyy">
</TextBlock>
</ScrollViewer>
</ContentControl>
<TextBlock Text="Footer"></TextBlock>
</StackPanel>
</Grid>
Hope it helps someone else (apart from being a bookmark for when I forget how I did it).
IT Bytes 
Glad I came across your posting. I thought there was something wrong with me. I am not sure how this feature of the textblock is supposed to be used “ScrollViewer.VerticalScrollBarVisibility” but it did not work for me also. Maybe something to do with the template and theming. Anyhow, adding a wrapping ScrollViewer did not completely solve it for me until I played with the maxHeight. Here is my code. Thank you for putting me on the right track.