Thanks for taking the time to report this bug.
This bug report isn't very useful because it doesn't describe the bug well. If you have time and can still reproduce the bug, please read http://bugzilla.gnome.org/bug-HOWTO.html and add a more useful description to this bug.

Thank you for your reply. I don't know what more information you would like, so I'll try to add what I think to be of value to this bug report.

Whenever you add an entry to the Start-up applications with the command 'nautilus' in it it will either be removed from the autostart applications or not be executed. Although it makes sense -- Nautilus is already started on log on -- this makes it impossible to automatically open a directory when logging in.

This is at least reproducible on GNOME 2.28 and most likely the previous releases as well.

@Sense sorry, but you understand what happening?
I don't know, and I think this description is a bit trivial

I agree that the description I provided is not very detailed. However, I'm just a bug triager, not a GNOME developer. All I know is that somehow all autostart entries for Nautilus are either removed or not executed. I suspect that the developers of GNOME Session know more about this and why it was done the way it is, but I don't know that.

not about whether you're a developer or not, the issue is if you can read this, because I think this says nothing

so if you add an entry with: nautilus /home/blah/Documents into the command entry, that is not going to be executed ? I'm just guessing since is not clear what you're trying to do, if that's the case it's working fine on latest GNOME 2.29 (available on Lucid) for me.

Sure!, if I follow the Pedro say work too for me, my case I use Ubuntu 9.10 with GNOME 2.28 and Nautilus version 2.28.1

On Lucid Lynx -- which according to apt-get policy gnome-session still runs 2.28.0 -- I can confirm that adding the command 'nautilus /home/blah/Downloads' to the list in System->Preferences->Startup Applications doesn't work.
Instead the entry is removed.

Here a better description since it's apparently impossible to edit the description:

When you try to let open a directory with Nautilus on log on by adding an entry to System->Preferences->Startup Applications the entry is removed from the list, which has as a consequence that you can't automatically open a Nautilus window on log in.

Steps to reproduce:
1) Go to System->Preferences->Startup Applications
2) Press Add
3) In the text field 'Name' fill something nice in
4) In the text field 'Command' fill any command starting with 'nautilus' in. This could be just 'nautilus', but also 'nautilus /home/blah/Directory'
5) Press Add and close Startup Applications
6) Log off
7) Log back in

Actual results:
The command isn't executed, the entry you added to Startup Applications is even removed.

Expected results:
The execution of the command inserted in the text field 'Command' on log in, even when it launches Nautilus.

Version & Platform:
This is reproducible on Ubuntu Karmic with:
gnome-session 2.28.0-0ubuntu5
nautilus 1:2.28.1-0ubuntu1

and on Ubuntu Lucid with:
gnome-session 2.28.0-0ubuntu7
nautilus 1:2.28.4-0ubuntu1

Created attachment 150237 [details]
ScreenShot System->Preferences->Startup Applications

Is work fine for me... look at the screenshot

On a clean account on Ubuntu 10.04 'Lucid Lynx' Alpha 2 I can confirm this as well with a new, clean account.

When entering the command 'nautilus' or the command 'nautilus /random/directory/' in the Startup application settings nothing happens on log in. However, when you enter the command 'nautilus /random/directory/', the command turns out to be shortened to 'nautilus' when you relaunch the Startup application settings dialogue.

this is still reproducible for you Sense?, for me work fine.

Closing this bug report as no further information has been provided. Please feel free to reopen this bug if you can provide the information asked for.
Thanks!