It's important to remember the destructive nature of Truecrypt's hidden volume function. Unless you use both keys to unlock the volume, opening the outer volume and allowing the OS to load will destroy the hidden volume that has your real files. It's meant to be a last-resort against a "Use this wrench and beat the password out of him" scenario.

I've only used it with a TrueCrypt volume as a file, where it works flawlessly (unless you continue to add files to the public volume). See http://www.truecrypt.org/docs/hidden-volume