Couldn't they just attempt to fill the "outside" partition up? I mean, let's say you have a 1TB partition with a 100GB hidden volume inside. What happens if somebody tries to write more than 900GB into the outside partition?
That said -- I would think that a random blob of data sitting around on the hard drive is still highly suspect. Aren't hard drives zeroed from the factory? And wouldn't any true "garbage" data be decidedly not random? (Even if it's compressed... you would still expect to find headers etc. somewhere.)
Take a look at http://www.truecrypt.org/docs/ - it is an interesting read.
When the police demand the key from you, you give them the one that unlocks the clean partition. Now, at this point it doesn't matter if they don't believe you, it doesn't matter if they know all about truecrypt and hidden partitions, there is no way for them to prove in a court of law or otherwise that there is a hidden partition there. You can just keep telling them "I gave you the password! I just wipe my free space with noise every night! It's just noise!" and you have plausible deniability.
As far as I'm aware this is only in theory. I'm not aware of any case of this actually being tested in court. But mathematically, it is apparently sound.