Which one does the OS care about?  blkid says the UUID is "787f1fa4-b010-4d77-a010-795b42884f56" while md insists its UUID is "4d96dd3b:deb5d555:7adb93cb:ce9182d9"

When in doubt, do we assume the OS takes the one from blkid? 

/dev/md0: UUID="787f1fa4-b010-4d77-a010-795b42884f56" TYPE="ext3"
[root@localhost ~]# mdadm -D /dev/md0
/dev/md0:
        Version : 0.90
  Creation Time : Fri May 28 15:48:07 2010
     Raid Level : raid1
     Array Size : 307202816 (292.97 GiB 314.58 GB)
  Used Dev Size : 307202816 (292.97 GiB 314.58 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Mon May 31 05:23:48 2010
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 4d96dd3b:deb5d555:7adb93cb:ce9182d9
         Events : 0.4

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

The real answer about the two sets of UUIDs

The blkid UUID above "787f1fa4-b010-4d77-a010-795b42884f56" is the correct one, that is what the OS will use to find the RAID array.

But mdadm has it's own "internal" UUID which is not used directly by the OS and is what you use in the mdadm.conf file eg:

ARRAY /dev/md0 level=raid1 num-devices=2 uuid=ffb40f02:91eebca2:cf946fc5:a655d98e
ARRAY /dev/md1 level=raid1 num-devices=2 uuid=0d4ae4bf:1b435759:c8f99160:83b906ee

This is another silly thing that programmers do without the implications of.  mdadm shouldn't call anything UUID when it is separate from the one that blkid and the OS recognize.  Maybe it should be called mduuid or something else to avoid confusion.