Dedup also helps with big media files that are identical across many users.

I'm pretty sure Bitcasa has said they avoid uploading known duplicates at all, which suggests the hash (either pre- or post- encryption) is shipped up first.

Cross account deduplication has nothing to do with making improvement client side, it's all about keeping storage costs down for the provider.

The only type of deduplication that matters to consumers is account specific deduplication, and that saves no data for the provider unless you charge for non deduplicated storage.

Per-block dedupe, I bet, so it'll get benefits at sub-file levels.

Though given that the vast majority (by volume) of customer data will be people's illegal downloads, their scheme effectively reduces to being a copy of usenet.

the biggest win by far is media files and other large assets like game files or large data sets that many, many people will have duplicate copies of. Even with a pretty low installed base 98%-100% of people's iTunes storage will be deduped. OS files are a big win too, really very very little (by block volume) on most people's drives is unique.