Still (in server x64 beta2) partitioning & raid building do not work correctly during installation.
I had 3 identical HDs, with identical /sda1 /sdb1 /sdc1 of 100MB each in the
beginning of the corresp. disk. Trying to build RAID1 with 2 active mirrors /sda1 and /sdb1
and spare /sdc1, the partitioner does not see (does not offer) to use /sdc1
as a spare. And there's no way to force it to, except adding a spare after the install.