It seems that there is a problem compiling the Linux version of Truecrypt v4.2a on the 2.6.18 series of kernels. A patch has been posted in the Truecrypt forums (you need to be logged-in to read), but, although this patch resolves the compilation error, there have been reports that the program has problems unmounting the encrypted volumes and releasing the loop devices. So, truecrypt is a no-go for Fedora 6 at the moment, at least until a new version that works flawlessly is released. An alternative way to create encrypted file containers pretty much like Truecrypt is by using dm-crypt and LUKS. More information about this method, can be found in another article of mine.
The TrueCrypt on 2.6.18 kernels by George Notaras, unless otherwise expressly stated, is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.