PHP will NOT display this on my dev machine: Warning: session_start()...

I am running into a strange problem and I hope someone might have an
idea why this is happening.

My installation of PHP will *NOT* display the warning message below on 
my development machine where it should display it (sample code at the 
bottom).
Warning: session_start() [function.session-start]: Cannot send session
cache limiter ....

After receiving a bug report from a customer I tested my code on a XAMPP
setup and, sure enough, it displayed the warning message.
But on my machine, I can't find a message in my php log, it is as if 
this problem does not even exist (on my dev machine).

My dev setup is:
OS: ARCH 64bit (about a month out of date)
PHP Dev stuff:
   Apache/2.2.14 (Unix) mod_ssl/2.2.14 OpenSSL/0.9.8k DAV/2 SVN/1.6.6
   PHP/5.3.1 with Suhosin-Patch
   xdebug-2.0.5-2-x86_64

php.ini
   error_reporting = E_ALL | E_STRICT
   display_errors = On
   display_startup_errors = On
   log_errors = On
   html_errors = On
   ....

phpinfo() confirms that these settings are in effect
	display_errors	On	On
	error_reporting	32767	32767

So does anybody have any clue as to what could be causing this problem 
of not getting a warning message?

Here is sample code:
<p>The warning should be below this line</p>
<?PHP session_start(); ?>
<p>The warning should be above this line</p>

Which should produce the message below between the lines:
Warning: session_start() [function.session-start]: Cannot send session
cache limiter - headers already sent ( output started at <file_name> on
line 2 )

but on my machine all I get is this in html source of the output:
<p>The warning should be below this line</p>
<p>The warning should be above this line</p>

thx
-- 
John
Staat heißt das kälteste aller kalten Ungeheuer.  Kalt lügt es auch; und
diese Lüge kriecht aus seinem Munde: 'Ich, der Staat, bin das Volk.'
[Friedrich Nietzsche]