something that could work as a mini-vacume. Something that you could use instead of a claw? just “suction” a plastic ball to pick it up? how much power would this drain? i know it would take a fairly smooth receptor surface…
Something like a miniature Toilet Plunger would be quite passive…
We’ve actually got some of the exact thing you’re talking about lying around in our old FRC stuff. It’s basically a suction cup with a hole out the back for a pneumatic hookup. It’s quite possible and from what the people who actually got to use it tell me, it works pretty well.
And it works with Compressed Air, NOT Vacuum, think about the Mechanics for a moment. 
FRC Team 957 looked at the Suction Cup for a solution to the 2007 Rack N’ Roll challenge.
that’s actually pretty much what i was thinking… although now that you put it that way, it’s kind of comical to think of a robot running about with a toilet plunger attached to it. 
That does sound impossible doesn’t it.
Either they used an air compressor running backwards to create suction (reversed polarity on the ones we’ve got) or they had a component of some sort that created suction using compressed air. I forget which it was, but the thing did run on a pneumatic system somehow.
Bernoulli’s principle: Bernoulli's principle - Wikipedia and http://www.youtube.com/watch?v=jxXPBOstpmE