[1] http://en.wikipedia.org/wiki/Deniable_encryption
[2] http://www.truecrypt.org/docs/plausible-deniability
[3] https://www.schneier.com/blog/archives/2008/07/truecrypts_de...
Could that be mitigated by having TC occasionally randomly write random data to unused blocks?
You have to consider the adversary. Are you a non-targeted individual who want to keep some things private from a spurious search by law enforcement or border agents, then simply the ability to boot up and not have sensitive stuff and not have an obvious encrypted partition lying around will do the trick.
If, on the other hand, you're targeted by someone with a strong reason to believe that you are hiding stuff on your computer, maybe even someone who will break into your apartment every day to image your computer, you're gonna have a bad time (and a keylogger on your computer, but that's a different kettle of fish). Basically, evidence of the existence of your hidden partition can leak out into the real world, not because of bad cryptography, but because you're human.
Its worth pointing out that his concerns seem to apply only to using Truecrypt to encrypt a portion of a drive, as opposed to whole-disk encryption.