This can indeed be achieved with a custom cell template. Here is what you need to do:

1. Copy the following Style resource to an accessible location within your project.

[code]<Style x:Key="BusyCellStyle" TargetType="{x:Type p:CellPresenter}">
     <Style.Triggers>
           <DataTrigger Binding="{Binding StyleKey, RelativeSource={RelativeSource Self}}" Value="Simple">
                 <Setter Property="Template">
                       <Setter.Value>
                             <ControlTemplate TargetType="{x:Type p:CellPresenter}">

                                   <Grid DataContext="{Binding Path=., RelativeSource={RelativeSource TemplatedParent}}" Margin="{Binding CellStyle.Margin}">
                                         <Border Margin="-4,0,0,0" Width="4" Background="Red" HorizontalAlignment="Left" />
                                         <Border BorderBrush="{Binding CellStyle.BorderBrush}" BorderThickness="{Binding CellStyle.BorderThickness}" Background="{Binding Background}" />
                                   </Grid>

                             </ControlTemplate>
                       </Setter.Value>
                 </Setter>
           </DataTrigger>

     </Style.Triggers>
</Style>[/code]
The above style is very similar to the default style of Timetable cells except that it adds a small red border to the left of the cell. We will use this style for the cells representing 'busy' time intervals.

Note that the namespace 'p' in the above XAML is defined as 'http://mindfusion.eu/scheduling/wpf'.

2. Handle the Calendar.CellPresenterCreated event and use an implementation similar to the following for the event handler:

[code]private void OnCellPresenterCreated(object sender, CellPresenterEventArgs e)
{
     // Perform a check if the cell is busy
     bool isBusy = false;
     if ((e.CellPresenter.Resource.Name == "Resource #1" ||
           e.CellPresenter.Resource.Name == "Resource #2") &&
           e.CellPresenter.StartTime.Hour > 12)
     {
           isBusy = true;
     }

     if (isBusy)
     {
           Style busyCellStyle = FindResource("BusyCellStyle") as Style;
           if (busyCellStyle != null)
                 e.CellPresenter.Style = busyCellStyle;
     }
}[/code]
The above code checks if the created CellPresenter corresponds to a busy cell and if so, associates our custom Style with it. The busy check in your case will be more complicated. In this sample we mark as busy all hours past 12:00 PM for the first two resources.

Let me know if this helps.

Regards,
Meppy

Just wondering: did you create the above xaml with Expression Blend, or did you do this by hand from visual studio?