As far as deduplication goes, I'm pretty sure both Mozy and Crashplan use client side dedupe over secure tunnels, which is now fairly common in the backup space so I wouldn't expect this to be any different.
On our Enterprise Backup platform at work we get roughly 20:1 dedupe ratios, so they'd be looking for some disk savings too.
File de-duplication on a per user/client where you're not de-duplicating across users is good. It wouldn't save anywhere near as much disk space as de-duplication across users though, 20:1 for just within per user data sounds pretty good/high.
On Dropbox if two different users store the same file in their respective accounts, Dropbox will only actually store a single copy of the file on its servers. To be able to do this the server must have access to the unencrypted data which means potentially Dropbox could be given a court order to disclose which of its users have stored a particular file.
Long story short: Use Truecrypt or Bitlocker on any folders/files you want to remain private before uploading to any of these cloud storage services, unless you know for sure they aren't using file de-duplication across users server side.