UBUNTU is now updated to 12.04. Ubuntu software center does not have qgis. therefore, qgis need to be installed indirectly.
Install QGIS 1.6.0 (Quantum Geographic Information System) in Ubuntu from PPA
Quantum GIS (QGIS) is a cross-platform Geographic Information System and it supports numerous vector, raster, and database formats and functionalities. QGIS lets you browse and create map data on your computer. It supports many common spatial data formats (e.g. ESRI ShapeFile, geotiff). The current version QGIS 1.6.0 releases with many bug fixes, features and enhancements. QGIS 1.6.0 available on Windows, MacOS X and Linux.
To install: open terminal and enter the following scripts.
sudo apt-get install add-apt-repository
sudo add-apt-repository ppa:ubuntugis/ubuntugis-unstable
sudo apt-get update
sudo apt-get install qgis
loading may take sometimes, be patient and wait till it ends; After installation it upgrades automatically to 1.7.4
check http://www.webcoz.com/install-qgis-1-6-0-quantum-geographic-information-system-in-ubuntu-from-ppa/ for further reference.