At some point I created a backup file on Windows 7 in a TrueCrypt volume container that was formatted with NTFS. I’m not sure why I originally formatted it with NTFS, but I think it was something to do with long filenames or a very deep directory nesting. Whatever. The point is, after upgrading to Lion, TrueCrypt would no longer mount this volume.
I did this several months ago, so I can’t remember what I originally did to get this setup, but apparently there’s something called MacFuse that adds support for additional filesystems, like NTFS on Mac. The version that I originally installed no longer works on Lion, so you need a different version that does – there’s a post here with a link to a version that works.
To get it working again, I uninstalled MacFuse and TrueCrypt, installed the version of MacFuse from above, reinstalled TrueCrypt, and not everything works.