Synaptic is no longer in the desktop task since tasksel 2.43: http://packages.debian.org/changelogs/pool/main/t/tasksel/current/changelog#version2.43
* Add kpackage to kde-desktop. * Move synaptic to gnome-desktop.
The idea of having each desktop provide its package manager was good, but kpackage was very far from matching Synaptic. It was even removed from KDE later, with the result that installing task-kde-desktop no longer causes the installation of a graphical package manager, as reported in http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=586982
There is still nothing that matches Synaptic today.
Furthermore, we now have LXDE and Xfce, which are also not preinstalling any graphical package manager. So I think Synaptic should be preinstalled on all DE-s... except that in Squeeze, GNOME itself does not preinstall Synaptic. Instead, it preinstalls GNOME PackageKit.
I asked about this, specifically about the situation for GNOME, in https://lists.debian.org/debian-devel/2012/03/msg00829.html
That message has some more relevant information.
There was one reply from Jon Dowland, who thinks "Synaptic is probably quite rightly not part of either [GNOME or KDE], but [agrees] that a "default desktop" via install should provide a GUI package manager." There were however no suggestions of alternatives.
It seems nobody thinks it's best to just install GNOME PackageKit.
Adding synaptic to task-desktop will effectively cause the preinstallation of both Synaptic and GNOME PackageKit on GNOME. I do not consider that a big problem, but gnome-core could stop depending on gnome-packagekit if we think it's best.