[CentOS] cgroups not working?

Hi,
I'm trying to use cgroups to limit the apache webserver but this doesn't 
seem to work as far as I can tell.
I modified cgconfig.conf like this:

group webserver {
         cpu {
                 cpu.shares="250";
         }
}

and addes this to cgrules.conf:
*:httpd         *               webserver

Looking into the "webserver" cgroup shows that the pid's of the httpd 
processes are correctly assigned:
$ cat /cgroup/cpu/webserver/tasks
6191
6193
6194
6195
6196
6197
6207

Yet when I run a php file with a dummy loop I get a process with 100% 
cpu usage until the script times out which is not what I expect.
Am I doing something wrong or why doesn't the cpu.shares setting seem to 
take?

Regards,
   Dennis