Most recent kernel where this bug did *NOT* occur:
2.6.18
The problem was introduced in 2.6.19-rc1

Distribution:
Debian unstable

Software Environment:
/dev/sda3 is /boot
/dev/sda4 is encryted with LUKS
# cryptsetup luksDump /dev/sda4
LUKS header information for /dev/sda4

Version:        1
Cipher name:    aes
Cipher mode:    cbc-essiv:sha256
Hash spec:      sha1
Payload offset: 2056
MK bits:        256
MK digest:      bd 91 fe fa 78 69 b0 c5 fd 67 e4 47 c8 0a 3c a4 26 a2 c3 63
MK salt:        1a 97 19 1e c9 2c bf ea e0 7e 9c b2 e3 4e 6d b9
                11 2d ca 29 d1 b6 1e d6 2a f9 eb 76 2c 86 e6 46
MK iterations:  10
UUID:           9af63e63-c81d-4f21-a300-94ffe86c8ffc

Key Slot 0: ENABLED
        Iterations:             172941
        Salt:                   40 c9 0f fa a7 63 1b 63 bd 12 24 96 6e e3 e8 eb
                                20 c1 14 90 3f 79 71 4b 14 25 42 38 ab 73 9d 2b
        Key material offset:    8
        AF stripes:             4000

Problem Description:
2.6.18 and 2.6.19 both with the same kernel configuration.
2.6.18 boots just fine. 2.6.19 fails in the initramfs after entering the
password in "cryptsetup luksOpen" with the following error:
---
device-mapper: table: 254:0 : crypt: Error allocating crypto tfm
device-mapper: ioctl: error adding target to table
device-mapper: device doesn't appear in the dev hash table
Failed to setup dm-crypt key mapping
---
This was manually copied via paper so it might not be 100% correct.