I'm looking for an industrial strength cloud file storage system. It will be used by multiple people in a startup.

Our requirements:

• Transparent file system access: files and folders in the file system must be able transparently access (read and write) files in the cloud; files must be synchronized whenever network access is available and buffered otherwise. The system must be usable by non-technical people.
• Access control: we need to control who can access which files, at least on a very coarse basis. e.g., the developers will be able to access the system design documents, only the corporate folks can access recruiting documents, and only management can access certain corporate documents. Dropbox provides this via Sharing folders, but that's not adequate, if I understand it correctly, because there's no authentication of the sharing user. so the cloud service should have a notion of an account (our startup) with multiple users with distinct credentials and rights for each user
• Clients: it must be accessible from Macs and PCs; I would hope that it supports Linux (e.g., Ubuntu) too
• Security: it must provide robust security
• Backup: the cloud service must reliably backup the files
• Versioning: change version history, is a big plus, but not required
• Not free: we're willing to pay for the service

So far, we've reviewed the following, albeit not completely thoroughly:

• Dropbox: has all except 1) Access control, which is provided via Sharing folders, but that's not adequate, if I understand it correctly, because there's no authentication of the sharing user. and 2) Security, as discussed here http://www.economist.com/blogs/babbage/2011/05/internet_security and here http://blog.dropbox.com/?p=821.
  • Windows Live Mesh, has all except 1) Clients, only supporting Windows 7 and OS X.
  • SpiderOak has all, except 1) Transparent file system access, which is only available for 1 user.
  • Amazon Cloud, doesn't offer 1) Transparent file system access
  • Rackspace Cloud Drive has all except 1) Access control and 2) Versioning

I'll gladly include any clarifications or additional systems the community provides.

Arthur