Nathaniel, 
What you described is exactly what I have also been experiencing.  If I called dmraid it then activated the partition, but it seemed every time I went through he process, it un-activated the dmpartion. 

So as a "fix" I tried to install directly to my disk not using my mapper device, but this did not work either.  Syslog error I got was:
...
May  4 03:11:36 ubuntu kernel: [  976.555661] end_request: I/O error, dev sdb, sector 268435416
May  4 03:11:36 ubuntu kernel: [  976.555695] sd 5:0:0:0: [sdb] Unhandled error code
May  4 03:11:36 ubuntu kernel: [  976.555697] sd 5:0:0:0: [sdb] Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK
May  4 03:11:36 ubuntu kernel: [  976.555699] sd 5:0:0:0: [sdb] CDB: Read(10): 28 00 57 54 66 e8 00 00 08 00
May  4 03:11:36 ubuntu kernel: [  976.555712] end_request: I/O error, dev sdb, sector 1465149160
May  4 03:11:36 ubuntu kernel: [  976.555747] sd 5:0:0:0: [sdb] Unhandled error code
May  4 03:11:36 ubuntu kernel: [  976.555749] sd 5:0:0:0: [sdb] Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK
May  4 03:11:36 ubuntu kernel: [  976.555751] sd 5:0:0:0: [sdb] CDB: Read(10): 28 00 57 54 66 e8 00 00 08 00
May  4 03:11:36 ubuntu kernel: [  976.555757] end_request: I/O error, dev sdb, sector 1465149160
May  4 03:11:36 ubuntu ubiquity: ERROR: asr: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: ERROR: ddf1: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: ERROR: ddf1: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: ERROR: hpt37x: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: ERROR: hpt45x: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: ERROR: isw: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: last message repeated 2 times
May  4 03:11:36 ubuntu ubiquity: ERROR: jmicron: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: ERROR: lsi: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: ERROR: nvidia: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: ERROR: pdc: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: last message repeated 48 times
May  4 03:11:36 ubuntu ubiquity: ERROR: sil: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu ubiquity: ERROR: via: reading /dev/sdb[Input/output error]
May  4 03:11:36 ubuntu kernel: [  976.575038] sd 5:0:0:0: [sdb] Unhandled error code
May  4 03:11:36 ubuntu kernel: [  976.575040] sd 5:0:0:0: [sdb] Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK
May  4 03:11:36 ubuntu kernel: [  976.575043] sd 5:0:0:0: [sdb] CDB: Read(10): 28 00 57 54 66 e8 00 00 08 00
May  4 03:11:36 ubuntu kernel: [  976.575050] end_request: I/O error, dev sdb, sector 1465149160
May  4 03:11:36 ubuntu kernel: [  976.575096] sd 5:0:0:0: [sdb] Unhandled error code
May  4 03:11:36 ubuntu kernel: [  976.575098] sd 5:0:0:0: [sdb] Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK
May  4 03:11:36 ubuntu kernel: [  976.575110] sd 5:0:0:0: [sdb] CDB: Read(10): 28 00 57 54 66 e8 00 00 08 00
May  4 03:11:36 ubuntu kernel: [  976.575116] end_request: I/O error, dev sdb, sector 1465149160
...

Matt