With Ubuntu 5.10 there already has been a special version of Ubuntu for servers.
For the upcoming version 6.04 a newly founded server team plans to further optimize the server edition of Ubuntu. It is being planed to test server hardware with Ubuntu in depth. Also the quality of the kernel shall be ensured to establish Ubuntu as a reliable server platform. IBMs DB2 has already been certified with Ubuntu.
Ubuntu 6.04 will be released in April 2006 and brings a longer support period. Canonical plans a commitment to security updates for three years for the desktop version and 5 years for the server version. Currently Canonical is looking after some new members for their Server team.