To get media files looping in the February CTP, you need to wire up a media timeline and manually catch the media_ended event, using the Controller to start the video again.  The RepeatBehavior property does not seem to work.  Assume you have some XAML like this:

<Button Click="play" Width="100" Height="100">play</Button>
<MediaElement Name="me" Width="100" Height="100"/>

To play and have the media loop, do this:

private void play(object sender, EventArgs e)
{
    MediaTimeline mt = new MediaTimeline(new Uri("mymedia.wmv", UriKind.Relative));
    MediaClock mc = mt.CreateClock();
    me.MediaEnded += new RoutedEventHandler(me_MediaEnded);
    me.Clock = mc;
    me.Clock.Controller.Begin();
}

void me_MediaEnded(object sender, RoutedEventArgs e)
{
    me.Clock.Controller.Begin();
}