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There are two services that I know of which offer truly secure online storage, Wuala and Spideroak. Jungle Disk can be fairly secure, as well. The former two use Java clients, and the latter is native.
Personally, I use Wuala and am pleased with it. There is one hole, though, in that it's possible to state whether a known file is stored in your account; if that's a problem, you could store it in a zip. That's good enough for me, so I use it for synchronization, backup, storage, and file sharing.