I checked.
But there may be more to it.
It always worked fine for me (including the OnlyShowIn or  NotShowIn keys).
Lately, after an xserver upgrade, gdm3 and SLiM started not to log into LXDE or IceWM here, only to Xfce.
I eventually tracked it down to gnome-settings-daemon; removing it solved the issue (but no theming in gdm3).
I only discovered that after reinstalling. It's in this current 'incarnation' that the system-wide autostart files aren't working.
But I noticed that Debian has a lxde-settings-daemon package (see http://packages.debian.org/sid/lxde-settings-daemon ) and there's no such a thing in the git repository.
I guessed that maybe the current lxde-common and/or lxsession have included its functionalities.
Or maybe not and I had it installed from the repo before (I may install it later to test).
It's all a bit confusing as some issues are overlapping and could be caused by Debian sid bugs (specially with these gnome components), maybe for me having built LXDE from upstream or whatever.
I'm just mentioning this because I was about to ask about this lxde-settings-daemon Debian package.
I'm still learning from experience.

Regards.

On Tue, May 10, 2011 at 7:29 PM, Julien Lavergne <gilir@ubuntu.com> wrote:
On Tue, 10 May 2011 14:14:53 -0300
Sergio Cipolla <secipolla@gmail.com> wrote:

> I noticed LXDE seems not to be loading the apps at /etc/xdg/autostart
> I has LXDE built from git and it was working fine in this respect then I
> reinstalled the system, built LXDE again and noticed this. Since it's
> lxsession that takes care of this and it was recently changed maybe it's a
> bug.
As far as I know, it's still the case. You should check if the desktops files in /etc/xdg/autostart don't have a key like OnlyShow=GNOME;KDE

Regards,
Julien Lavergne