Dear Baptiste.
I used cobbler to manage my CentOS 5.x repos and kickstarts, but Spacewalk is now a more mature project and so I start to use it, but as you know it works with rhn-* tools.
Do rhn tools becomes available in future versions of CentOS (CentOS 5.6, CentOS 6...) ?
If you Kickstart your systems through Spacewalk (that's what it's meant for) you can associate a Channel containing the Tools to the Kickstart Profile.
The packages in there will be available during installation phase (before system registration) as well. So there is not need for having them in CentOS.
Kind Regards Marcus