I did another test. First of all I shutdown and powered down the server properly. Then I unplugged one device and started the computer again. mdadm detected the missing device. After a couple of minutes I re-attached the SATA cable. mdadm didn't make an attempt to integrate the partitions.

Here is an output of mdadm -D for the MD device and -E for all three partitions. /dev/sdc1 is the partition of the disk that was missing during boot. The 'Array State : AAA' of /dev/sdc1 looks suspicious to me.

# mdadm -D /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Tue Apr 17 15:09:30 2012
     Raid Level : raid1
     Array Size : 487412 (476.07 MiB 499.11 MB)
  Used Dev Size : 487412 (476.07 MiB 499.11 MB)
   Raid Devices : 3
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Mon Apr 23 14:11:48 2012
          State : clean, degraded 
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : vlserver:0  (local to host vlserver)
           UUID : b3b367ee:f2f4f4c0:c22cdc7b:4ac1ea2c
         Events : 89

    Number   Major   Minor   RaidDevice State
       3       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
       2       0        0        2      removed

# mdadm -E /dev/sda1
/dev/sda1:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : b3b367ee:f2f4f4c0:c22cdc7b:4ac1ea2c
           Name : vlserver:0  (local to host vlserver)
  Creation Time : Tue Apr 17 15:09:30 2012
     Raid Level : raid1
   Raid Devices : 3

 Avail Dev Size : 974824 (476.07 MiB 499.11 MB)
     Array Size : 974824 (476.07 MiB 499.11 MB)
    Data Offset : 24 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : caee6e67:ea75cde1:bd3ec883:f6c4f709

    Update Time : Mon Apr 23 14:11:48 2012
       Checksum : 4ea9d4d9 - correct
         Events : 89


   Device Role : Active device 0
   Array State : AA. ('A' == active, '.' == missing)

# mdadm -E /dev/sdb1
/dev/sdb1:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : b3b367ee:f2f4f4c0:c22cdc7b:4ac1ea2c
           Name : vlserver:0  (local to host vlserver)
  Creation Time : Tue Apr 17 15:09:30 2012
     Raid Level : raid1
   Raid Devices : 3

 Avail Dev Size : 974824 (476.07 MiB 499.11 MB)
     Array Size : 974824 (476.07 MiB 499.11 MB)
    Data Offset : 24 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : 8ee52fbe:6f8dae0c:ffc93d08:bfcd4870

    Update Time : Mon Apr 23 14:11:48 2012
       Checksum : bb12772a - correct
         Events : 89


   Device Role : Active device 1
   Array State : AA. ('A' == active, '.' == missing)
# mdadm -E /dev/sdc1
/dev/sdc1:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : b3b367ee:f2f4f4c0:c22cdc7b:4ac1ea2c
           Name : vlserver:0  (local to host vlserver)
  Creation Time : Tue Apr 17 15:09:30 2012
     Raid Level : raid1
   Raid Devices : 3

 Avail Dev Size : 974824 (476.07 MiB 499.11 MB)
     Array Size : 974824 (476.07 MiB 499.11 MB)
    Data Offset : 24 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : e7df883f:f6bda45d:973bbdad:364290be

    Update Time : Mon Apr 23 14:05:59 2012
       Checksum : 812786b4 - correct
         Events : 75


   Device Role : Active device 2
   Array State : AAA ('A' == active, '.' == missing)