nohup is ignored if not executed in a terminal

Binary package hint: lxdm

I'm using Ubuntu 11.04 dev with GNU coreutils 8.5. I have written a script in ~/test.sh (chmod 755) with the following content:

#!/bin/sh
nohup sleep 1000 > /dev/null 2>&1

If I execute ~/test.sh in a terminal all is working fine. I can logout without that sleep 1000 will hang up. But if I try to execute this file with a filemanager (in my case PCManFM) the sleep 1000 command is executed but sleep 1000 will hang up if I logout.