I've been running my Truecrypt unRAID for 10 months now and it is 100% stable and reliable. I have five 1.5 TB containers formatted with RaiserFS, which I mount with a shell script at boot (truecrypt /mnt/disk1/data-01 /data/data-01). I would imagine I could use an NTFS container, too, if I loaded NTFS-3G but since I have no need for it, I haven't tried it. All the necessary packages are installed from /boot/custom prior to mounting by the same script, including the TrueCrypt binary (which I downloaded from repository.slacky.eu along with all the other packages). To share the encrypted data from mount points I have my own smb.conf and smb.shares, which are copied and enabled (smbcontrol smbd reload-config) at boot as well. Since I never stop the array, Bubba's concern about smb.shares getting overwritten is not a problem for me.
The key to get all this working was a custom kernel and modified source files, which I had to build using a full Slackware distro. It's been a while now so I don't exactly remember what I had to change but kapperz's error message about device-mapper missing from the kernel sounds very familiar.
To smino's point, the reason is that I consider my network to be fairly secure. If, however, someone would e.g. steal my server he wouldn't be able to read the data in the encrypted containers (without torturing the passwords out of me or cracking the encryption, in either case I'm likely to be dead. Or maybe use a cold boot attack or a (remote) keylogger).