If you are using Ubuntu 12.10 and using a Sony Xperia S probably you are facing issues to get its internal memory mounted on your computer.
Fortunately someone at Google faced this same issue and decide to fix it (ironically Google and Samsung created this issue when they decide to use MTP on Galaxy Nexus S for the first time).
The software is called go-mtpfs and it is easy to install and use:
sudo add-apt-repository ppa:webupd8team/unstable sudo apt-get update sudo apt-get install go-mtpfs
You can also install a unity plugin:
sudo apt-get install go-mtpfs-unity