If you only have Macs, using an encrypted sparse bundle disk image is pretty simple, and changed files result in a small delta for dropbox to sync. It's built in and you'll be up and running in a couple minutes: http://matthew.mceachen.us/blog/free-easy-encrypted-storage-...
it is only simple if you always manage not to open such a sparse bundle on another Mac before it has been fully synced after being used on another Mac. Otherwise, you will mess up your sparse bundle sooner or later.
A service for syncing and sharing with built-in encryption might therefore me more convenient for most users. Spideroak and Wuala are two examples.
