It's a pitty that root is not in repository anymore (applies for Ubuntu 11.04). Here is a way
HOW-TO install root on Ubuntu Natty (x86):

DEPENDENCIES:
    sudo apt-get install gcc g++ make binutils subversion dpkg-dev
                     libx11-dev libxpm-dev libxft-dev libxext-dev
    OPTIONAL DEPENDENCIES: (I've this installed) sudo apt-get install cmake
                     gsl-bin libgsl0-dev
                     liblzma2 liblzma-dev
                     libgmp3c2 libgmp3-dev
                     libpcre3 libpcre3-dev
                     zlib1g zlib1g-dev
HOW TO INSTALL:

wget ftp://root.cern.ch/root/root_v5.34.00.source.tar.gz
tar -xzvpf root_v5.34.00.source.tar.gz
cd root

./configure
make
sudo make install                     ## sudo needed at least for for /etc/root
sudo

cd root
source bin/thisroot.sh
root                                  
.Q                                  // to quit root