Description of problem:
Today I installed F18 Alpha on my desktop.
I had one partition (a btrfs partition on sda5) which i wanted to keep (it is my old /home)
I configured everything in manual partitioning, removing my old root(/dev/sda4) and swap(/dev/sda3) and adding new ones.
when i continued the installer i saw a status message about formating /dev/sda3 as btrfs, which I did not ask for (i asked for an ext4 partition and swap).
a quick check with parted confirmed what i was afraid of:

  Anaconda deleted my sda5 and recreated it with the same parameters (size/fs) on sda3 (but of course not the data). without me having asked for a change to this partition.