The TCHunt program, whose website this link points to, is quite interesting. It seems to exist to simply point out something I always felt was self-evident, yet I commonly see misunderstood:

"Q. Why write a program such as TCHunt?

A. To demonstrate that while encrypted volumes may be indistinguishable from random data created in one specific fashion that the volumes themselves can be easily distinguished from most other files on your system. Many people insist that their encrypted volumes are undetectable. I hope TCHunt will convince them otherwise, before they learn this fact the hard way. More importantly, you should never claim that an encrypted volume with a mp3 file extension (or whatever) is a corrupt file, etc. While that explanation may seem plausible to an average person, it will not stand up to forensic or legal scrutiny. Data corruption does not resemble AES encrypted data. If disclosing the location of your encrypted volumes may lead to legal issues, then say nothing and contact a competent lawyer."

Of course, as he notes, it can't tell the difference between regular and hidden volumes, but once again: the hidden volumes will still stick out like a sore thumb and clearly be encrypted data. While plausible deniability is technically maintained, believable deniability will always be harder.