On Mac OS X, when I mount a TrueCrypt volume from a file container while logged in as user A, I can then switch to another user B and view the mounted volume's content (e.g. in the Finder, or in a Terminal.app session). I believe this is a bug, as the content of the TrueCrypt volume should remain private. I don't know enough about the underlying issues to lay the blame on any one in particular (Mac OS X, TrueCrypt, FUSE?), but what I definitely can say is that I cannot trust my Mac to be left alone while a TrueCrypt volume is still mounted.
This is how my mounted volumes' mount points look like inside a Terminal.app session. As you can see, the TrueCrypt volume
PRIVATE is mounted with permissions that make it wide open for any user to snoop around inside.
nargothrond:~ --> ls -l /Volumes/
drwxr-xr-x 1 patrick staff 8192 12 Dez 2010 BOOTCAMP
lrwxr-xr-x 1 root admin 1 29 Aug 21:17 Macintosh HD -> /
drwxrwxrwx 1 patrick staff 16384 31 Dez 1979 PRIVATE
I have reported this issue on the TrueCrypt website in September 2009. I never received a reply. Today I double-checked whether the problem is still there with the latest version of TrueCrypt (7.1a): Yes, it is! In case anyone wonders: I am using Mac OS X 10.6.8.