How to install qt 4.8 and QtCreator from repository ?
The coming Precise Pagoling is having the Qt 4.8.1, at the moment. The PP will be released: https://wiki.ubuntu.com/PrecisePangolin/ReleaseSchedule -> April 26th.
The Kubuntu developers ( http://www.kubuntu.org/ )have backported the Qt 4.8 to the Oneiric Ocelot: https://launchpad.net/~kubuntu-ppa/+archive/backports .
From the Personal Package Archives for Ubuntu: https://launchpad.net/ubuntu/+ppas?name_filter=qt4-x11
I didn't find Qt 4.8 in ubuntu official repository. You should check if the debian package works (I'm not sure it's a good idea to use debian packages, but it might works) : http://packages.debian.org/fr/experimental/libqtcore4
Beside that there is an installer on the official Qt Website (I prefer that solution, seems less risky) : http://qt.nokia.com/downloads/