The information comes via a Twitter conversation with EmmanueGT...
@MuscleNerd will there be a unlock software coming out for the 3gs 3.1.3 baseband 05.12.01 when the new iPhone 4g comes out??
@EmmanueGT yep..so don't sell or give away your 3GS :) (at same time, remember not to accept Apple FW upgrades in future)
So to reiterate, don't jump to update when iPhone OS 4.0 comes out!