I think this is really smart. Think how insane garage sales can be now when people can accept credit card payments. Or being able to split checks at a restaurant (when the waiter/waitress wont do it) by charging the others at the table while you pay the restaurant. Sure it looks like Square is losing money on this, but I think it was on HN where I read that Square was already processing $1mil a day. I'd also assume that this $10 credit won't be around forever.
Either way, the simplicity of Square is what will make consumers interested.