Backpack? I'm not quite sure what you mean by that. The only thing I can think of is what we call canister style vacuums up here (or at least in my family).

Sadly, I don't know of any. We have one that has to be 20 years old now, and it still [mostly] does its job.

I know it won't be a winner, but my parents recently got one of these.
Advantages:
-small, extremely light-weight, cordless
-detachable "dust buster"

Disadvantages:
-not the strongest suction, but has brushes on the upright part
-its use here is to clean up loose dirt on wood and stone floors, very little on carpet, so I don't know what it would be like in a whole house