If you are using encrypted files and try to create an encrypted file container or volume you will get the error message " test failed at truecrypt::encryption test:testxts:831 " under snow leopard. there isn't a fix or good work around yet and there are reported problems with mounting as well. I hope this soon changes.
Why use encryption? Well if files are stored in such a container and somebody gets your notebook it won't be possible to open the files without knowing the password.
There's no need to actually use True Crypt for a volume creation - mac os has built in support for that! open the disk utility for that :