IOMega used to make a NAS with support for foldershare (the dropbox predecessor that was sold to Microsoft). That was 6 years ago - before it's time I'd guess.
The device 'Time Machine' by Apple does exactly that.
No it doesn't, it backsup your Macs over wifi, and that's all it does. It won't access web services, which is they key point I was trying to make. The Time Capsule, and all home backup devices I can name, make the soon-to-be-outdated assumption that your data exists on a hard drive you own.
Well more elaborately your mac can access all web services and download all data which can then be wirelessly backed up by the time machine.
For example my Dropbox folder is automatically synced on my mac which is then automatically backed up by the time machine.
I hope they've thought of this and I've just missed it. They won't be able to sell USB based backup drives for much longer.