Weight distribution dictates traction, quickness to rotate, and significantly impacts packaging. The Ferrari 599 is a LONG car for only a 2-seater. Much of this length probably came from their desire to achieve the rear-weight bias (must lengthen the hood and move the engine back and well behind the axle). The M3 starts as a 3-series, and thus by nature a more practical car that is not too long (180 inches) yet still can carry 4 folks and some luggage. Because they used a pretty small, light, and compact V8 they were able to position the engine pretty far back in the bay. Then they used additional light weight tech and weight tricks (battery in trunk, as mentioned) to nearly attain the 50/50 goal.
Keep in mind if someone orders some options (rear sun shade, ampified high end audio with amp in the trunk, etc) these options will add weight to the rear and may actually reach the 50/50 goal. Card and Driver weighed it as 51.9% front, 48.1% rear, which is pretty darn close to 50/50. If you want perfect 50-50 just throw a friend in the trunk.
For racing purposes I believe a slight rear bias is preferred (see Forumla 1 = mid engine, rear weight bias). But mid-engined cars are a lot less practical from a packaging perspective.