We have an App for iOS that renders much text: http://bit.ly/IekVmU
We use CoreText in iOS to render the pre-formatted text and allow the user to change formatting, font-size and font face.
We are trying to port it to Windows Phone but I'm not sure if there's a substitute for CoreText in Windows Phone.
WP7 allows you to create text from Runs, for example:
<TextBlock TextWrapping="Wrap">
<TextBlock.Inlines>
<Run FontWeight="Bold">I am Bold</Run>
<Run>I am not</Run>
</TextBlock.Inlines>
</TextBlock>
This allows you to create blocks of text with mixed format.