Author: Jonas Bilinkevicius 

I want my application to go into 'sleep mode' when the screen saver has been 
started.

Answer:

1   unit U1;
2   
3   interface
4   
5   uses
6     Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs, 
7   StdCtrls;
8   
9   type
10    TForm1 = class(TForm)
11      Label1: TLabel;
12      procedure FormCreate(Sender: TObject);
13    private
14      { Private declarations }
15      procedure AppMessage(var AMessage: TMsg; var Handled: Boolean);
16    public
17      { Public declarations }
18    end;
19  
20  var
21    Form1: TForm1;
22  
23  implementation
24  
25  {$R *.DFM}
26  
27  {Get screen saver messages}
28  
29  procedure TForm1.AppMessage(var AMessage: TMsg; var Handled: Boolean);
30  begin
31    if ((WM_SYSCOMMAND = AMessage.message) and (SC_SCREENSAVE = AMessage.wParam)) then
32    begin
33      Handled := True;
34      Label1.Caption := 'Warning! Screen saver detected. Top clearance required.';
35      Handled := False;
36    end;
37  end;
38  
39  {On form create}
40  
41  procedure TForm1.FormCreate(Sender: TObject);
42  begin
43    Application.OnMessage := AppMessage;
44  end;
45  
46  end.