ext4 on encrypted devices, such as dm-crypt/LUKS, is supported. What is not supported are encryption filesystems that are 'filesystem overlays', such as ecryptfs.
(Since I was using ZFS, I am still debating whether to stay with Dropbox after November's filesystem apocalypse.)
FYI, you can still use ecrypt fs with dropbox. Put the encrypted store within your dropbox, and mount it outside your dropbox. From the dropbox point of view, you have thousands of files with gibberish as names.
