What the OP don't understand is that there are services like that already (i.e. [1]). You want something more drobxy but supposedly secure?! Go for SpiderOak!!! Plenty of solutions around you could even buy Amazon S3 or Glacier storage yourself, make a gpg and use dump/tar/whatever-utility-you-like encrypt and upload them yourself using a script (or set of scripts).
The difference is, I trust Colin. Aside from djb he's one of the few people on this planet I would trust to write crypto/security software and get it right.
SpiderOak? Only heard about them because they're "hip" on here, given the state of technology in the average startup, I wouldn't trust them with my data ever.
So you want the hip part of Spider-oak and the quality of TarSnap. That's fine to ask, but these things very rarely play well together.
PS. Let's not be harsh on SpiderOak though, I never used it for backup purposes and haven't read about any security breaches or privacy concerns about it anyway.
[1] http://www.haystacksoftware.com/arq/