I'm replacing the default Button template's ContentPresenter with a TextBlock, so the text can be trimmed when it's too long.

Works fine in WPF. In Silverlight the text gets pushed to one edge and cut off on the left, even when there's space on the right:

Template is nothing special, just replaced the ContentPresenter with the TextBlock:

            <Border x:Name="bdrBackground" 
            BorderBrush="{TemplateBinding BorderBrush}" 
            BorderThickness="{TemplateBinding BorderThickness}" 
            Background="{TemplateBinding Background}" />

        <Rectangle x:Name="rectMouseOverVisualElement"
            Opacity="0">
            <Rectangle.Fill>
                <SolidColorBrush x:Name="rectMouseOverColor" 
                    Color="{StaticResource MouseOverItemBgColor}"/>
            </Rectangle.Fill>
        </Rectangle>
        <Rectangle x:Name="rectPressedVisualElement" 
            Opacity="0" 
            Style="{TemplateBinding Tag}"/>

        <TextBlock x:Name="textblock" 
            Text="{TemplateBinding Content}" 
            TextTrimming="WordEllipsis"
            TextWrapping="NoWrap"
            HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" 
            Margin="{TemplateBinding Padding}" 
            VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>


        <Rectangle x:Name="rectDisabledVisualElement" 
            Opacity="0" 
            Style="{StaticResource RectangleDisabledStyle}"/>

        <Rectangle x:Name="rectFocusVisualElement" 
            Opacity="0" 
            Style="{StaticResource RectangleFocusStyle}"/>              

    </Grid>
</ControlTemplate>  

How do I fix this?

More info: With the latest comment about HorizontalAlignment, it's clear that SL's implementation of TextTrimming differs from WPF's. In SL, TextTrimming only really works if the text is aligned left. SL isn't smart enough to align the text the way WPF does. For instance:

WPF button:

SL button with the textblock horizontalalignment = left:

SL button with textblock horizontalalignment = center: