I have some LUKS encrypted volumes, both internal SATA and external USB disks.  Hald detects the presence of the encrypted volumes but not the plaintext volumes when the device mapping is set up.
When I attach the external disk, a "crypto_LUKS" volume appears in dolphin's places bar and I can successfully setup a mapping by entering the passphrase.  But as with the fixed disks that are mounted at boot, the plaintext volume is never detected.
According to the logs the slave device is not found and from a glance at hotplug_event_begin_add_blockdev() in hald/linux/blockdev.c I suspect that this might be caused by the volume.fstype property (= "crypto_LUKS") being set on a fakevolume (whatever that is) and not the actual block device, while the slave device enumeration in hotplug_event_begin_add_blockdev() doesn't consider the fakevolume.

hal version: 0.5.11, same problem applies to 0.5.12rc1
kernel: 2.6.28
distribution: gentoo

Log snippet of one of the devices:
[I] blockdev.c:874: block_add: sysfs_path=/sys/devices/virtual/block/dm-10 dev=/dev/mapper/permanent is_part=0, parent=0x00000000
[I] blockdev.c:915: Looking in /sys/devices/virtual/block/dm-10/slaves for Device Mapper
[I] blockdev.c:926:  /sys/devices/virtual/block/dm-10/slaves/. -> /sys/devices/virtual/block/dm-10/slaves
[I] blockdev.c:958: Couldn't find slave volume in devices
[I] blockdev.c:926:  /sys/devices/virtual/block/dm-10/slaves/.. -> /sys/devices/virtual/block/dm-10
[I] blockdev.c:958: Couldn't find slave volume in devices
[I] blockdev.c:926:  /sys/devices/virtual/block/dm-10/slaves/sdd -> /sys/devices/pci0000:00/0000:00:1f.2/host3/target3:0:0/3:0:0:0/block/sdd
[I] blockdev.c:964: Done looking in /sys/devices/virtual/block/dm-10/slaves
[I] blockdev.c:970: Ignoring hotplug event - no parent
[W] blockdev.c:1473: Not adding device object

relevant lshal output:
udi = '/org/freedesktop/Hal/devices/storage_serial_SATA_ST31500341AS_9VS13VFP'
  block.device = '/dev/sdd'  (string)
  block.is_volume = false  (bool)
  block.major = 8  (0x8)  (int)
  block.minor = 48  (0x30)  (int)
  block.storage_device = '/org/freedesktop/Hal/devices/storage_serial_SATA_ST31500341AS_9VS13VFP'  (string)
  info.capabilities = {'storage', 'block'} (string list)
  info.category = 'storage'  (string)
  info.parent = '/org/freedesktop/Hal/devices/pci_8086_2922_scsi_host_2_scsi_device_lun0'  (string)
  info.product = 'ST31500341AS'  (string)
  info.udi = '/org/freedesktop/Hal/devices/storage_serial_SATA_ST31500341AS_9VS13VFP'  (string)
  info.vendor = 'ATA'  (string)
  linux.hotplug_type = 3  (0x3)  (int)
  linux.sysfs_path = '/sys/devices/pci0000:00/0000:00:1f.2/host3/target3:0:0/3:0:0:0/block/sdd'  (string)
  storage.automount_enabled_hint = true  (bool)
  storage.bus = 'scsi'  (string)
  storage.drive_type = 'disk'  (string)
  storage.firmware_version = 'CC1H'  (string)
  storage.hotpluggable = false  (bool)
  storage.lun = 0  (0x0)  (int)
  storage.media_check_enabled = false  (bool)
  storage.model = 'ST31500341AS'  (string)
  storage.no_partitions_hint = false  (bool)
  storage.originating_device = '/org/freedesktop/Hal/devices/computer'  (string)
  storage.partitioning_scheme = 'none'  (string)
  storage.removable = false  (bool)
  storage.removable.media_available = true  (bool)
  storage.removable.media_size = 1500301910016  (0x15d50f66000)  (uint64)
  storage.requires_eject = false  (bool)
  storage.serial = 'SATA_ST31500341AS_9VS13VFP'  (string)
  storage.size = 1500301910016  (0x15d50f66000)  (uint64)
  storage.vendor = 'ATA'  (string)
  volume.ignore = true  (bool)

udi = '/org/freedesktop/Hal/devices/volume_uuid_8d3cf090_84ba_424f_acff_0a80118b7f3d'
  block.device = '/dev/sdd'  (string)
  block.is_volume = true  (bool)
  block.major = 8  (0x8)  (int)
  block.minor = 48  (0x30)  (int)
  block.storage_device = '/org/freedesktop/Hal/devices/storage_serial_SATA_ST31500341AS_9VS13VFP'  (string)
  info.capabilities = {'volume', 'block'} (string list)
  info.category = 'volume'  (string)
  info.interfaces = {'org.freedesktop.Hal.Device.Volume.Crypto'} (string list)
  info.parent = '/org/freedesktop/Hal/devices/storage_serial_SATA_ST31500341AS_9VS13VFP'  (string)
  info.product = 'Volume (crypto_LUKS)'  (string)
  info.udi = '/org/freedesktop/Hal/devices/volume_uuid_8d3cf090_84ba_424f_acff_0a80118b7f3d'  (string)
  linux.hotplug_type = 3  (0x3)  (int)
  linux.sysfs_path = '/sys/devices/pci0000:00/0000:00:1f.2/host3/target3:0:0/3:0:0:0/block/sdd/fakevolume'  (string)
  org.freedesktop.Hal.Device.Volume.Crypto.method_argnames = {'passphrase', ''} (string list)
  org.freedesktop.Hal.Device.Volume.Crypto.method_execpaths = {'hal-luks-setup', 'hal-luks-teardown'} (string list)
  org.freedesktop.Hal.Device.Volume.Crypto.method_names = {'Setup', 'Teardown'} (string list)
  org.freedesktop.Hal.Device.Volume.Crypto.method_signatures = {'s', ''} (string list)
  storage.model = ''  (string)
  volume.block_size = 512  (0x200)  (int)
  volume.fstype = 'crypto_LUKS'  (string)
  volume.fsusage = 'crypto'  (string)
  volume.fsversion = '2'  (string)
  volume.ignore = true  (bool)
  volume.is_disc = false  (bool)
  volume.is_mounted = false  (bool)
  volume.is_mounted_read_only = false  (bool)
  volume.is_partition = false  (bool)
  volume.label = ''  (string)
  volume.linux.is_device_mapper = false  (bool)
  volume.mount_point = ''  (string)
  volume.num_blocks = 2930277168  (0xaea87b30)  (uint64)
  volume.size = 1500301910016  (0x15d50f66000)  (uint64)
  volume.uuid = '8d3cf090-84ba-424f-acff-0a80118b7f3d'  (string)