This posting is older than 6 months and can contain outdated information.
Felix,
This option has been removed and is no longer available.
You can do the exactly same thing with your system's Login Items,
though (=> System Preferences.app > Users > Login Items).
Dragging a volume there will mount it upon system startup.
Thanks. I have done that (after your suggestion), and it indeed worked. But, it also made a finder window pop up every login (when it "runs" the network volume, it mounts it and opens a finder window inside it). It's kind of annoying.
Then, I searched a little and found a tip about using automator to do it. With little effort I created a script in automator that does only that, mount that specific network volume. Then I run this script on my login items.
I just tought I should leave this comment here to help others with the same problem.
The URL is http://www.macosxhints.com/article.php?story=20070202190047133