Have also looked forward to this, itīs one of the major features that have been missing.
This together with Btrfs ability to change RAID levels, add new disks to an existing array on-the-fly and rebalance is really nice. Here Btrfs has a real edge over ZFS which is very static once created.
According to the Btrfs FAQ there are also plans for per-subvolume and per-file RAID levels which will become handy in some cases