S/390 has been an officially supported Debian architecture since the release of Debian 3.0 (woody).
For instructions on installing Debian, see the Installation Guide. Unfortunately for Sarge it is not possible to ipl the installation system off an installation CD, but several other installation methods are supported. Support to ipl off an installation CD has been added back in the version of the installer that will be released with Etch.
The primary liaison between Debian and the Linux for S/390 developers at IBM is Gerhard Tonn.
The following other people also contributed to the Debian port to s390, in alphabetical order:
The build servers for the S/390 architecture are hosted by Millenux, the distributor of the Think Blue distribution for S/390, and the Linux Community Development System (LCDS).
The wanna-build DB is on auric.debian.org. To be able to work with the database, please ask Ryan Murray to be added to the wb-s390 group.
If you would like to help, you should subscribe to the debian-s390 mailing list. To do this, send a message containing the word "subscribe" as the subject to debian-s390-request@lists.debian.org, or use the mailing list web page. You can also browse and search the mailing list archive.