After a fresh installation of Ubuntu 13.10 I noticed that there was no steam in the Ubuntu Software Center! Well, that's mot a problem since steam can be installed through the deb file on their website. However, just out of curiosity, I would like to know if it will be available in the software center anytime soon!
Steam is still offered in the USC according to this in Saucy. Maybe you need to activate/add the proper repositories for the USC show it to you:
sudo add-apt-repository http://repo.steampowered.com/steam/ steam sudo apt-get update