The following patch fixes the problem:

== 
$ cvs diff -u ext/date/
cvs diff: Diffing ext/date
Index: ext/date/php_date.c
===================================================================
RCS file: /repository/php-src/ext/date/php_date.c,v
retrieving revision 1.43.2.45.2.51
diff -u -r1.43.2.45.2.51 php_date.c
--- ext/date/php_date.c 12 Jul 2007 18:59:05 -0000      1.43.2.45.2.51
+++ ext/date/php_date.c 6 Nov 2007 17:40:24 -0000
@@ -1613,7 +1613,7 @@
 static void date_object_free_storage_timezone(void *object TSRMLS_DC)
 {
        php_timezone_obj *intern = (php_timezone_obj *)object;
-
+       timelib_tzinfo_dtor(intern->tz);
        zend_object_std_dtor(&intern->std TSRMLS_CC);
        efree(object);
 }
cvs diff: Diffing ext/date/lib
cvs diff: Diffing ext/date/tests

== 
Seems it was simply forgotten to free the timezone in DateTimeZone's destructor.

Because ->tz is an internal pointer (and not a zval) memory_get_usage() will not report a memory increase, you'll have to use Task Manager / top / ps to see the effect

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.