This is strange as I was able to install Synaptic on Kubuntu 11.10
from the Muon Software Centre, which means it is in the repositories.
I use the server for Canada but I checked and it is in the main server
too. Check your server settings and your spelling.

It is there. I use it everyday.

Check your sources list. Have you added outside sources that might be
interfering with Synaptic being installed?

I would then try sudo apt-get clean to clear the cache and try again.

might be a good idea to run an 'sudo apt-get update' to make sure
apt has a list that is recent, too.

Maybe package name changed?

Use 'apt-cache search synaptic' to see a list of packages containing that
string.

Also make sure software center is not active when you are trying to use
apt. At least in the past they both used some of the same resources and
would conflict.

Could be ubi's repositories are temporarily down or your sources are not
properly enabled too.

It is a good idea to check the Software Centre. It is still called synaptic.