When the iPad 3Gs are out, someone will try putting in a cut but normal sim card into an unlocked unit.
Until then, we won't know if we can put our current sims (after cutting) into an unlocked iPhone 4G and if the phone will work.
Either:
it won't acknowledge an old sim
it acknowledges it but Apple have done something extra and it won't work
it acknowledges it and it's just like the regular unlocked iPhones (albeit after having to cut your own sim card to fit)
Until then, we won't know if we can put our current sims (after cutting) into an unlocked iPhone 4G and if the phone will work.
Either:
it won't acknowledge an old sim
it acknowledges it but Apple have done something extra and it won't work
it acknowledges it and it's just like the regular unlocked iPhones (albeit after having to cut your own sim card to fit)