Ubuntu Home Server (UHS) will be an edition of the Ubuntu operating system which allows users to administer their home network. With Ubuntu Home Server you will be able to store all your music, songs and pictures in one central location, to access your files over the internet and to backup all the computers in your house.
The goal is to have a working release available with the upcoming Gutsy Gibbon release of Ubuntu (or perhaps Gutsy+1 at the latest).
The UHS project is community driven. It is not (yet) an official Ubuntu project.
The idea of a Home Server Edition of Ubuntu was recently outlined in this thread on the official Ubuntu forums:
Ubuntu is a leading free operating system for use on personal computer and servers: Ubuntu.com