A change introduced in Silverlight 5 is that a ListBox ( or any itemscontrol ) will now scroll a selected item into view if it's off screen.
The rest of this article proves that.

Demonstration


Create a new Silverlight project.

Markup


Paste the following markup  into MainPage:

<Grid x:Name="LayoutRoot" Background="White">
    <StackPanel>
        <ListBox Name="lb" Height="100">
            <ListBoxItem>a</ListBoxItem>
            <ListBoxItem>b</ListBoxItem>
            <ListBoxItem>c</ListBoxItem>
            <ListBoxItem>d</ListBoxItem>
            <ListBoxItem>e</ListBoxItem>
            <ListBoxItem>f</ListBoxItem>
            <ListBoxItem>g</ListBoxItem>
            <ListBoxItem>h</ListBoxItem>
            <ListBoxItem>i</ListBoxItem>
            <ListBoxItem>j</ListBoxItem>
            <ListBoxItem>k</ListBoxItem>
        </ListBox>
        <Button Height="24" Click="Button_Click">Select k</Button>
    </StackPanel>
</Grid>

That's a listbox containing 11 items and sized so they can't all fit in view.
Plus a button.  You're about to paste some code in for that click event,,,

Code Behind

We need a little code to select something.

private void Button_Click(object sender, RoutedEventArgs e)
{
    lb.SelectedIndex = 10;
}

The collection in that listbox is zero based so index 10 will be "k".

Results


Spin her up for a ride with an f5.
You see:


Nothing is selected and only the first 4 items are visible.
Click the button.
You should now see:



All that code does is select the item but Silverlight worked out it's not in the visible part of the ListBox and brought it into view.

Any reader more familiar with WPF may well be turning green with envy about now.
In WPF you'd need to use BringIntoView and even then the thing wouldn't have focus so it'd be light grey unless you did some more coding.