The Toyota MR2, Honda NSX, Lotus Elise, Ferrari 308 and 328, and Lamborghini Miura are all mid-rear engined cars, with engines mounted transversely. I don't know what Fiero you're looking at, but my engine is ahead of the rear axle.
A rear engined car has the engine entirely behind the rear axle: Chevy Corvair, Porsche 911, VW Bug, etc.
The benefit of a mid-engined design is to concentrate the mass between the axles and minimize the car's moment of inertia. Most mid-engined cars, including ones with longitudinally mounted engines, have a rear weight bias.
My father's Z is front engined, but with him in it all four wheels are within 50 lbs of each other. Perfect weight distribution is not the goal of putting the engine in the middle, and is easily attained with other configurations.
[This message has been edited by Will (edited 06-26-2001).]