We have made the change because Git offers many benefits over Subversion. Basically all large Open Source projects have preceded us in this move. For more details about the why, how and where see this presentation [2].
A detailed Git HowTo [3] has been added to the ROOT website.
To checkout the ROOT sources, clone in Git parlance, do:
git clone http://root.cern.ch/git/root.git [4]For the rest, in respect to building ROOT, nothing has changed.