Bug#604033: Bug report: Installing Squeeze on a Intel Matrix BIOS RAID1 volume
I've experienced the same problems. After using the daily build testing netinst cd on 7/7/2011 (http://cdimage.debian.org/cdimage/daily-builds/daily/arch-latest/amd64/iso-cd/debian-testing-amd64-netinst.iso) the partition dialog showed the same problems described by Delton.
The usual dialog with the four options "1) guided partitioning... 4) manual partitioning" were not shown if a Raid1 partition is configured in the BiosRaid. This step seems to be skipped and the next step where you usually edit the partitions is shown, but no drives or partitions are listed there. Everything works fine, if the partition configured in the biosraid is a Raid0 partition or a non raid partition. It may be possible to make some more tests or provide more information, if this will be helpful to you.