How about a bitcoin coinstar? Seems like a great idea to me. You put coins in there, converts it to Bitcoins.
Not to be a smartass or anything, but isn't that desribing an ATM(BTM?) pretty much?
But I know what you mean. Bring a bucket of coins, and get bits in return. I'm pretty sure the people dispatching BTM's have minimums in place, can't dispense dust amounts for the amount their paying to be in that public place.