Ubuntu (and Debian) doesn’t have support for the SATA RAID (also called FakeRAID since some of the functionality is provided by software) controllers being shipped on recent motherboards (The only distros to support these currently are Gentoo and Fedora Core 5). The software to handle FakeRAID arrays in Linux is dmraid.
Dmraid is currently not integrated into the Debian installer or the Ubuntu Live-CD installer. Therefore, one cannot directly install Ubuntu onto such RAID arrays. Previously, one had to install Ubuntu on to a separate partition and then debootstrap Ubuntu. However the Dapper Live-CD can be used to easily install Ubuntu Dapper onto such RAID arrays