Where should the bt-applet.desktop file go?

/etc/xdg/autostart seems to work for GNOME but not for KDE.
/usr/share/autostart seems to work for KDE but not for GNOME.

http://standards.freedesktop.org/autostart-spec/autostart-spec-latest.html
http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html#variables

$ echo $XDG_CONFIG_DIRS
/etc/xdg.d/kde:/etc/xdg

So when KDE doesn't find it in /etc/xdg/autostart, it's buggy?

It should go in /usr/share/gnome/autostart (just like the autostart files for
g-p-m, nm-applet and g-v-m) as I believe you _only_ want to start it
automatically when running a GNOME session. This is a GNOME specific directory
for things that should _only_ be started when starting a GNOME session.

If you want it to run for all XDG compliant desktops (which I don't think you
want to do as it's GNOME specific) look at the specifications mentioned in
comment 2.

Despite the silly name, I do want it to start in all sessions. My understanding
of the referenced specification was that I should therefore put the .desktop
file in /etc/xdg/autostart. Which didn't work for KDE -- even, iirc, when I
omitted the 'OnlyShownIn: GNOME;' line :)

I'll retest.

Then I'd use /etc/xdg/autostart and file a bug to get KDE fixed. 

than,

do you know what the status of autostart compliance is in kde right now?

I wonder if this should be closed, based on what I see on my rawhide system:

[mclasen@localhost ~]$ rpm -q bluez-gnome
bluez-gnome-0.6-1.fc7

So was this package formally approved?  Currently it's still blocking FC-NEW,
and there's no indication of approval in any of the comments.