Convert an existing Debian installation to Ubuntu without reinstalling from scratch.
While the usual approach to switching Linux distributions is to erase your disk and reinstall from scratch, Ubuntu is based on Debian, so it's possible to switch directly from one to the other—provided you're willing to spend time tweaking and fixing obscure problems. It's even possible to have your computer track both Debian and Ubuntu simultaneously and install packages from one or the other at will.
The naive solution to converting an existing Debian system to Ubuntu is to just edit your /etc/apt/sources.list to switch all the Debian archive references to the equivalent Ubuntu archives and then upgrade all packages. That's a good start, but unfortunately it will most likely leave you with a fairly broken system because many of the libraries in Debian will have newer versions than their equivalents in Ubuntu, so it's necessary to do a bit more work to end up with a usable system.
Edit your /etc/apt/sources.list to comment out all the Debian archives and add entries for the Ubuntu archives. If you need to generate an Ubuntu sources.list file, you can use the "source-o-matic" tool available online at http://www.ubuntulinux.nl/source-o-matic, or you can just put in some basic entries as a starting point:
deb http://archive.ubuntu.com/ubuntu dapper main restricted deb http://security.ubuntu.com/ubuntu dapper-security main restricted
Then update the ...