I use kernel 2.6.23.1.
When I tried to compile truecrypt 4.3a I got the following error:

Checking build requirements...
Building kernel module... .../truecrypt-4.3a-source-code/Linux/Kernel/Dm-target.c: In function ‘dm_truecrypt_init’:
.../truecrypt-4.3a-source-code/Linux/Kernel/Dm-target.c:659: error: too many arguments to function ‘kmem_cache_create’
[...]
Error: Failed to build kernel module

Fixed it following the instructions I found somewhere else (http://forums.fedoraforum.org/showthread.php?p=873476):
Remove the last 'NULL' in line 659 of Dm-target.c in truecrypt.../Linux/Kernel.