Well, right now nothing stops someone from creating a system where:
A sends 1.00000001 to B B sends 1.00000000 back to A
Net result is a micro-payment and no processing fee.
… unless B started with zero bitcoins. Then B is stuck; she can’t send 1.0 back, because doing that would cause a 0.00000001 bitcoin ‘change’ transaction, which would trigger the 0.01BTC fee, which they can’t pay (because they only have 1.0000000001).