Synaptic is a graphical package management program for apt. It provides the same features as the apt-get command line utility with a GUI front-end based on Gtk+.
We all knew this was going to happen at some point since there is a lot of work going on for the Ubuntu Software Center, but here's a confirmation: Paul Tagliamonte, an Ubuntu LoCoCouncil and BeginnersTeam Council member confirmed in a comment on Reddit that Synaptic will soon be removed from Ubuntu.
It's not yet sure, but it seems Synaptic will be removed starting with Natty Narwhal (Ubuntu 11.04):
Synaptic is going bye-bye soon. [...]
It should have been removed on Lucid, but it slipped ( SC had too few features ). I don't think it's off the default for maverick, but it should be gone by Natty.
Of course, you will still be able to install Synaptic via the Ubuntu Software Center or using "apt-get", but it won't be installed by default anymore (just like it happened with Aptitude in Maverick).