1. After updating from F21 to F22 via yum and booting into the F22 kernel, the system hangs at "Reached target Basic System".

2. Added "rd.shell rd.initdebug" to the linux cmdline; this shows that /dev/md0 (or /dev/disk/by-uuid/5029707e...) is not automatically assembled anymore.

3. Booted into the old F21 kernel (still works), re-recrated the initrd, to be on the safe side:
> dracut -v -N -f --mdadmconf --kver=4.0.3-300.fc22.x86_64

4. Problem remains.

5. Waiting for the rd.shell, issuing a simple "mdadm --assemble /dev/md0" brings /dev/md0, and a subsequent ctrl-d lets me boot F22.

More information:

> cat /proc/mdstat 
Personalities : [raid1] 
md0 : active raid1 sda2[0] sdb2[1]
      235238268 blocks super 1.1 [2/2] [UU]
      bitmap: 2/2 pages [8KB], 65536KB chunk

unused devices: <none>

> cat /etc/mdadm.conf
# mdadm.conf written out by anaconda
MAILADDR root
AUTO +imsm +1.x -all
ARRAY /dev/md0 level=raid1 num-devices=2 UUID=c7fa38f7:f04850a1:d70d735c:c198f383

> mdadm --detail /dev/md0
/dev/md0:
        Version : 1.1
  Creation Time : Thu Jun  9 19:25:58 2011
     Raid Level : raid1
     Array Size : 235238268 (224.34 GiB 240.88 GB)
  Used Dev Size : 235238268 (224.34 GiB 240.88 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

  Intent Bitmap : Internal

    Update Time : Tue May 19 17:29:06 2015
          State : clean 
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : myhostname:0  (local to host myhostname)
           UUID : c7fa38f7:f04850a1:d70d735c:c198f383
         Events : 58001

    Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       1       8       18        1      active sync   /dev/sdb2


RPMs:
kernel-4.0.3-300.fc22.x86_64
dracut-041-10.fc22.1.x86_64
mdadm-3.3.2-1.fc22.x86_64

Please let me know if you need additional information.