encrypted filesystem on pre-existing filesystem?

[Bill Wagner <bwagner@gory.org>]

I've got a few boxes running testing/unstable and I'd like to test
running encrypted filesystems on some of them.

My question is if it's possible to set up a pre-existing reiserfs or
ext3 filesystem for encryption.  From what I've seen, it's more or less
mounting it as loopback with losetup but it appears you've got to set up
the encryption before creating the filesystem.  Is it typically best to
use the 'AES' encryption with a extremely long pass phrase, BTW?

Thanks in advance,

Bill

PS:  Will this ever be an option available at install time (or is it
already and I just missed it?)