The problem is, of course, that this adds complexity, and get's in the way of one's workflow.
However, I realised yesterday that there's an easy way to do this.
~$ ecryptfs-setup-private ~$ ln -s ~/.Private ~/Dropbox/Private
Note the "." in the first path
Now anything you put into ~/Private will be automatically synced up to Dropbox. Very, very convenient way of doing this IMHO.
These instructions are very, very simplified, and the ecryptfs setup can be done in many other ways than using the setup-private script. Using a common encryption passphrase across your boxes, or not encrypting filenames, or using public/private key for the ecrtyptfs mount all should be investigated.
But the main point here is the symlink idea to the hidden encrypted folder. I don't recall hearing anyone suggest this before.