UCK is a tool that helps you customizing official Ubuntu Live CDs (including Kubuntu/Xubuntu and Edubuntu) to your needs. You can add any package to the live system, for example language packs, or applications.
An earlier blog post about UCK.
Reconstructor is a similar project :
Reconstructor is a Live CD creator for Ubuntu Linux.
It uses the Ubuntu Linux Live CD as a base, and then allows customization of boot screens (usplash), gnome settings, and software (you can also use the chroot environment to make other changes before creating the live cd).
the forum thread for Reconstructor
You can find yet another way to create a customized live cd on this wiki page.
Og Maciel blogged about this subject here.
Here’s a related forum thread.
Creating a customized Ubuntu cd is on my to-do list and it will be relatively easy thanks to all these great efforts.