Karsten Januszewski blogged about the system styles available for text in Windows Phone 7.
For example, I noticed the following XAML that referenced PhoneTextTitle1Style in one of the Visual Studio project templates:
<TextBlock x:Name="ListTitle" Text="{Binding LineOne}" Margin="-3,-8,0,0" Style="{StaticResource PhoneTextTitle1Style}"/>
I then wanted to figure out what all the system styles were. How to do it? Expression Blend!