I am using SL5 and Test Studio.  I Built a small SL app with some labels and a RadMaskedTextBox.  (See xaml below.)
 
<Grid x:Name="LayoutRoot" HorizontalAlignment="Center" Height="115" Width="185"> 
        <StackPanel Orientation="Vertical"> 
            <Border Style="{StaticResource GridCellDataStyle}"> 
                <sdk:Label Name="FirstName" Content="John" HorizontalAlignment="Stretch" FontSize="15" /> 
            </Border> 
            <Border Style="{StaticResource GridCellDataStyle}"> 
                <sdk:Label Name="LastName" Content="Doe" HorizontalAlignment="Stretch" FontSize="15" /> 
            </Border> 
            <Border Style="{StaticResource GridCellDataStyle}"> 
                <sdk:Label Content="{Binding Path=UserName}" HorizontalAlignment="Stretch" FontSize="15" /> 
            </Border> 
            <Border Grid.Row="1" Grid.Column="2" Style="{StaticResource GridCellDataStyle}"> 
                                <telerik:RadMaskedTextBox MaskType="None" Width="140" Value="{Binding CurrentQuantity, Mode=TwoWay}" IsReadOnly="True" IsEnabled="False" MaxLength="50"  /> 
            </Border> 
        </StackPanel> 
    </Grid>
 
I created a test in Test Studio and began recording.  However, I cannot build a verification on the RadMaskedTextBox.  The verification selector will highlight in red my other controls.  But not the RadMaskedTextBox.

To reproduce:
Sample Silverlight app is attached to the internal description.