Comment 13 for bug 543838

In the mean time, I've been able to circumvent this bug and finish the installation process normally (using the Alternate CD beta-1 amd64 ISO image) by:

1. creating the physical partitions (fdisk) using the debian-installer's partition manager

2. [CTRL+ALT+F2] creating the RAID array manually:
mdadm --create /dev/mdX --level=5 --raid-devices=4 --chunk=256 /dev/sd[abcd]X # I chose to use 256k chunk size

3. creating LVM volumes manually:
pvcreate /dev/mdX # NB: LVM seems to recognize the chunk size and align optimally, cf. pvs -o +pe_start
vgcreate vg.raid5 /dev/md2
lvcreate -n lv.root -L10G vg.raid5
lvcreate -n lv.var -L10G vg.raid5
lvcreate -n lv.tmp -L10G vg.raid5

4. [CTRL+ALT+F1] Click on "Go Back" in the debian-installer partition manager and click on "Partition Disks" again (partition layout is refreshed); the error is still there (and the /dev/mdX itself can not be used directly) BUT the logical volumes are there and available for use. At the end of the installation, the system reboot and starts flawlessly (the RAID configuration is created successfully by the debian-installer, despite the error displayed by 'parted'

5. Create any "big" logical volumes you need (see 3.) once rebooted

PS: even though LVM was used on top of RAID5, bonnie++ performances match what can be expected of the setup (120MB/s block write, 300MB/s block read).

Still a hack, but allows to go through the installation process. Hopes this helps