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Although offtopic, because not DIY, the answer, for now, for me, is "Wuala". http://www.wuala.com/
High quality java software, all content fully encrypted, sophisticated neatly designed access rights management (cryptree).
It's not open source, but otherwise really close to perfect.
I am in no way associated with the company (originally "Caleido", now merged into "Lacie").