I've been pouring concrete all my life, plus I'm a construction engineer.
If they're pouring your driveway and its raining or is about to rain, then it MUST be covered! The introduction of water to the finished surface at the setting stage will promote spalling of the surface.
Part of concrete finishing is being able to predict the weather and the onus is on the finishing company. Driveways are typically broom finish, which is one of the easiest finishes to place; so get those boys to get some rain protection over the concrete asap. Once its cured you can keep it moist for added strength, the slower it cures, the greater the strength.
If they haven't finished pouring, do what MILT said. Stop the pour halfway, leave the surface rough and pour the topping on a better sunny day. If you live in a region where rain is apparant, your finishers should know the game.
Taken just two days ago... my crew:
