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I'm planning to set up a large (probably 6-9Tb) raid-5 array on ubuntu using mdadm, lvm and ext3. Does anyone have any opinions or suggestions on the safety of using a Truecrypt volume for that amount of data with this setup?

I have had a windows Vista laptop drive using Truecrypt whole-disk (System) encryption fail once, however that time I was able to boot off an ubuntu live-cd, save an image of the entire disk to a server over the network using the excellent ddrescue (or was it dd_rescue?), then decrypt the image using truecrypt on the server. It took a long time but I was able to recover almost all of the files. I think I was lucky in that case. Things would be very different with 9Tb of data to copy! Of course, none of this was truecrypt's fault, but encryption certainly adds an extra step to data-recovery.

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RAID is not a backup. Provided you have proper backups of your TrueCrypt drive (or container file), you'll be fine, just like you were with Vista.

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+100 if I could. There are only three possibilities -- 1) You have a backup. 2) You don't care about your data. 3) You are an idiot. -- Your question rules out 2. So now you get to choose between 1 and 3. – David Schwartz May 2 '12 at 6:08
I'll keep a backup of the important stuff but the other 8.99Tb I would be annoyed, but not inconsolable if it was lost. I think that raid-5 with redundancy is good enough for me (I'll know if one drive fails), when compared to the alternative of paying for and setting up another 9Tb server. The question was just hopefully to tilt the odds in my favor if (when?) the worst happens. – localhost May 2 '12 at 6:35
The odds will always be in your favour if you have an excellent disaster recovery strategy. Back up your data, and test your backups. The rest is irrelevant. – user3463 May 2 '12 at 17:02
Agree on backups. Other than that, OP may choose increasing the RAID level at the expense of the free space. It is not uncommon for two drives in a RAID fail simultaneously or one after another, so personally I have chosen RAID6. Reducing the available RAID volume space also cheapens backups. :-) – minya May 7 '12 at 10:32
Sorry to bump an old thread, but to clarify RAID 6 isn't "5+1". There are no "mirror sets" (you're thinking RAID 51, although it's still not true that you'd lose that after any two discs drop). RAID 6 is exactly like RAID 5 (XOR'd parity data), but with an extra parity block (instead of 1 there are two). This means you have three "copies" of your data, and can therefor read and rebuild your array if you lose ANY two discs, regardless of which two they are in the RAID set. – devnul3 Oct 9 '13 at 1:41

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