LoginView.xaml

<UserControl x:Class="SCO.Ria.UI.Views.LoginView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:toolkit="http://schemas.microsoft.com/winfx/2006/xaml/presentation/toolkit"    
    xmlns:interactivity="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
    xmlns:local="clr-namespace:Caliburn.Micro.Focus;assembly=Caliburn.Micro.Focus"
    xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk" mc:Ignorable="d"
    d:DesignHeight="154" d:DesignWidth="468">

    <interactivity:Interaction.Behaviors>
        <local:TabNextBehavior/>
    </interactivity:Interaction.Behaviors>

    <toolkit:BusyIndicator x:Name="IsBusy">
        <toolkit:BusyIndicator.BusyContent>
            <TextBlock Text="Verificando ..." />
        </toolkit:BusyIndicator.BusyContent>
        <Grid Background="White">
            <Grid.RowDefinitions>
                <RowDefinition Height="40" />
                <RowDefinition Height="40" />
                <RowDefinition Height="100" />
                <RowDefinition Height="*" />
            </Grid.RowDefinitions>

            <Grid Grid.Row="0">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition />
                    <ColumnDefinition />
                </Grid.ColumnDefinitions>
                <TextBlock Text="Usuário:" Grid.Column="0" Margin="5" />
                <TextBox x:Name="Model_Username" Grid.Column="1" Margin="5" Width="200" />
            </Grid>

            <Grid Grid.Row="1">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition />
                    <ColumnDefinition />
                </Grid.ColumnDefinitions>

                <TextBlock Text="Senha:" Grid.Column="0" Margin="5" />
                <PasswordBox x:Name="Model_Password"  Grid.Column="1" Margin="5" Width="200" />
            </Grid>

            <sdk:ValidationSummary Grid.Row="2" IsTabStop="False" />

            <StackPanel Grid.Row="3" Orientation="Horizontal" HorizontalAlignment="Center" Margin="5">
                <Button Content="Entrar" x:Name="LoginButton" Grid.Row="2" Width="100" Height="25" Margin="5" />
                <Button Content="Cancelar" x:Name="CancelButton" Grid.Row="2" Width="100" Height="25" Margin="5" />
            </StackPanel>
        </Grid>
    </toolkit:BusyIndicator>
</UserControl>

TabNextBehavior.cs

using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Interactivity;

namespace Caliburn.Micro.Focus {
    public class TabNextBehavior : Behavior<Control> {

        protected override void OnAttached() {
            base.OnAttached();

            this.AssociatedObject.KeyUp += AssociatedObject_KeyUp;
        }

        protected override void OnDetaching() {
            base.OnDetaching();

            this.AssociatedObject.KeyUp -= AssociatedObject_KeyUp;
        }

        void AssociatedObject_KeyUp(object sender, KeyEventArgs args) {

            if (args.Key == System.Windows.Input.Key.Enter) {

                DependencyObject parent = null;
                if (AssociatedObject is ChildWindow)
                    parent = ((ChildWindow)parent).Content as DependencyObject;
                else parent = AssociatedObject;

                parent.TabNext(); //extensin Method from VisualTreeExtensions.cs
            }
        }
    }
}

VisualTreeExtensions.cs

using System.Collections.Generic;
using System.Linq;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;

namespace Caliburn.Micro.Focus {
    public static class VisualTreeExtensions {
        public static T GetAncestor<T>(this FrameworkElement source) where T : class {
            var current = source;
            while (current != null) {
                if (current is T) return current as T;
                current = current.Parent as FrameworkElement;
            }
            return null;
        }

        public static List<T> FindControlsRecursive<T>(this DependencyObject container)
            where T : DependencyObject {
            if (container == null) return null;

            var foundControls = new List<T>();

            //for each child object in the container
            for (var i = 0; i < VisualTreeHelper.GetChildrenCount(container); i++) {
                var control = VisualTreeHelper.GetChild(container, i);
                if (control is T)
                    foundControls.Add((T)control);

                if (VisualTreeHelper.GetChildrenCount(control) > 0) {
                    foundControls.AddRange(FindControlsRecursive<T>(control));
                }
            }

            return foundControls;
        }

        public static Control FindControlByName(this DependencyObject container, string propertyName) {
            for (var i = 0; i < VisualTreeHelper.GetChildrenCount(container); i++) {
                var child = VisualTreeHelper.GetChild(container, i);
                var control = child as Control;
                if (control != null) {
                    if (control.Name == propertyName)
                        return control;
                }

                if (VisualTreeHelper.GetChildrenCount(child) > 0) {
                    return FindControlByName(child, propertyName);
                }
            }

            return null;
        }

        public static void TabNext(this DependencyObject parentElement) {

            //CSharpExtensions.Guard(() => parentElement != null, "parentElement");

            var fromControl = FocusManager.GetFocusedElement() as Control;

            if (fromControl == null)
                return;

            var children = parentElement.FindControlsRecursive<Control>()
                .Where(c => c.IsEnabled && c.IsTabStop && c.Visibility == Visibility.Visible)
                .ToList();

            if (children.Contains(fromControl)) {
                var thisIndex = children.IndexOf(fromControl);
                var targetIndex = thisIndex + 1;
                if (children.Count > targetIndex) {
                    var targetChild = children[targetIndex];
                    fromControl.Dispatcher.BeginInvoke(() => {
                        targetChild.Focus();
                        var txt = targetChild as TextBox;
                        if (txt != null) {
                            txt.SelectAll();
                        }
                    });
                }
            }
        }
    }
}

Original Question from StackOverflow http://stackoverflow.com/questions/1354903/auto-tab-in-silverlight-3#_=_

What timing! I need this. Is there a reason you put these into Caliburn's namespace?

Perfect! Thanks!