They are never seen because the only other probe to fly by Mercury, Mariner 10, only mapped about 40-45% of Mercury. MESSENGER will see the parts that have never before been seen. Additionally, Mercury is always too close to the Sun (in angular separation) to point Hubble toward it. So these really are going to be never before seen images. Not just new, crisper images of previously seen terrain.
Am I the only Slashdotter who looked at this and thought, "Of course they've never been seen, they haven't even been taken yet." Yes, yes, I know what they meant, but couldn't they have said what they meant instead of something dramatic but wrong?