RAID not auto-assembling?

I have a 3-disk RAID5, with a consistent superblock, and partitions
are all 0xFD.

The array isn't recognized by mdadm at boot or when I manually ask it
to assemble /dev/md0 without specifying partitions. The three
partitions are always /dev/sd[abc]1.

Output of -D /dev/md0 is:

/dev/md0:
        Version : 00.90.03
  Creation Time : Sun Jun 15 22:02:57 2008
     Raid Level : raid5
     Array Size : 1465143808 (1397.27 GiB 1500.31 GB)
  Used Dev Size : 732571904 (698.64 GiB 750.15 GB)
   Raid Devices : 3
  Total Devices : 3
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Tue Aug 26 01:03:31 2008
          State : clean
 Active Devices : 3
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

           UUID : eaa2f834:729baa7b:5b2687e9:8461c801
         Events : 0.512

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
       2       8       33        2      active sync   /dev/sdc1