Similar here. dmraid -ay silently fails and no /dev/mapper devices are created.  I can confirm that it works in gentoo with 2.6.23, but not in ubuntu hardy liveCD.

misc. output:

ubuntu@ubuntu:~$ sudo dmraid -ay -vvvv -dddd
WARN: locking /var/lock/dmraid/.lock
NOTICE: /dev/sdb: asr     discovering
NOTICE: /dev/sdb: ddf1    discovering
NOTICE: /dev/sdb: hpt37x  discovering
NOTICE: /dev/sdb: hpt45x  discovering
NOTICE: /dev/sdb: isw     discovering
NOTICE: /dev/sdb: jmicron discovering
NOTICE: /dev/sdb: lsi     discovering
NOTICE: /dev/sdb: nvidia  discovering
NOTICE: /dev/sdb: nvidia metadata discovered
NOTICE: /dev/sdb: pdc     discovering
NOTICE: /dev/sdb: sil     discovering
NOTICE: /dev/sdb: via     discovering
NOTICE: /dev/sda: asr     discovering
NOTICE: /dev/sda: ddf1    discovering
NOTICE: /dev/sda: hpt37x  discovering
NOTICE: /dev/sda: hpt45x  discovering
NOTICE: /dev/sda: isw     discovering
NOTICE: /dev/sda: jmicron discovering
NOTICE: /dev/sda: lsi     discovering
NOTICE: /dev/sda: nvidia  discovering
NOTICE: /dev/sda: nvidia metadata discovered
NOTICE: /dev/sda: pdc     discovering
NOTICE: /dev/sda: sil     discovering
NOTICE: /dev/sda: via     discovering
DEBUG: _find_set: searching nvidia_cecdfbfh
DEBUG: _find_set: not found nvidia_cecdfbfh
DEBUG: _find_set: searching nvidia_cecdfbfh
DEBUG: _find_set: not found nvidia_cecdfbfh
NOTICE: added /dev/sdb to RAID set "nvidia_cecdfbfh"
DEBUG: _find_set: searching nvidia_cecdfbfh
DEBUG: _find_set: found nvidia_cecdfbfh
DEBUG: _find_set: searching nvidia_cecdfbfh
DEBUG: _find_set: found nvidia_cecdfbfh
NOTICE: added /dev/sda to RAID set "nvidia_cecdfbfh"
DEBUG: checking nvidia device "/dev/sda"
DEBUG: checking nvidia device "/dev/sdb"
DEBUG: set status of set "nvidia_cecdfbfh" to 16
WARN: unlocking /var/lock/dmraid/.lock
DEBUG: freeing devices of RAID set "nvidia_cecdfbfh"
DEBUG: freeing device "nvidia_cecdfbfh", path "/dev/sda"
DEBUG: freeing device "nvidia_cecdfbfh", path "/dev/sdb"


ubuntu@ubuntu:~$ sudo dmraid -n
/dev/sdb (nvidia):
0x000 NVIDIA  
0x008 size: 30
0x00c chksum: 3487021370
0x010 version: 100
0x012 unitNumber: 1
0x013 reserved: 131
0x014 capacity: 781422720
0x018 sectorSize: 512
0x01c productID: MIRROR   372.61G
0x02c productRevision: 100
0x030 unitFlags: 0
0x034 array->version: 6553668
0x038 array->signature[0]: 1251801591
0x03c array->signature[1]: 132050278
0x040 array->signature[2]: 1018414995
0x044 array->signature[3]: 1056154375
0x048 array->raidJobCode: 0
0x049 array->stripeWidth: 1
0x04a array->totalVolumes: 2
0x04b array->originalWidth: 1
0x04c array->raidLevel: 129
0x050 array->stripeBlockSize: 128
0x054 array->stripeBlockByteSize: 65536
0x058 array->stripeBlockPower: 7
0x05c array->stripeMask: 127
0x060 array->stripeSize: 128
0x064 array->stripeByteSize: 65536
0x068 array->raidJobMark 0
0x06c array->originalLevel 129
0x070 array->originalCapacity 781422720
0x074 array->flags 0x0

/dev/sda (nvidia):
0x000 NVIDIA  
0x008 size: 30
0x00c chksum: 3487086906
0x010 version: 100
0x012 unitNumber: 0
0x013 reserved: 131
0x014 capacity: 781422720
0x018 sectorSize: 512
0x01c productID: MIRROR   372.61G
0x02c productRevision: 100
0x030 unitFlags: 0
0x034 array->version: 6553668
0x038 array->signature[0]: 1251801591
0x03c array->signature[1]: 132050278
0x040 array->signature[2]: 1018414995
0x044 array->signature[3]: 1056154375
0x048 array->raidJobCode: 0
0x049 array->stripeWidth: 1
0x04a array->totalVolumes: 2
0x04b array->originalWidth: 1
0x04c array->raidLevel: 129
0x050 array->stripeBlockSize: 128
0x054 array->stripeBlockByteSize: 65536
0x058 array->stripeBlockPower: 7
0x05c array->stripeMask: 127
0x060 array->stripeSize: 128
0x064 array->stripeByteSize: 65536
0x068 array->raidJobMark 0
0x06c array->originalLevel 129
0x070 array->originalCapacity 781422720
0x074 array->flags 0x0


ubuntu@ubuntu:~$ sudo dmraid -b
/dev/sdb:    781422768 total, "9QH0364K"
/dev/sda:    781422768 total, "9QH02C6Q"