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I was looking at one of these this last weekend and I noticed that, based on the dirt streamlines, airflow starts to spill off the hood maybe 2/3 of the way back (about even with the top of the wheel arches), and continues over the well-rounded hood-fender transition and onto the sides.
I don't know how much this contributes to the low drag number, but it seems like the same sort of thing a big hood-bulge might induce.
It appears that even with the relatively flat windshield (compared to, say, an old Saab), they've found a way to get the air to go (partly) around the sides of the passenger compartment instead of going over it. Seems like this would reduce lift (span-wise flow tends to do that) to a small degree as well.
The B5 Passat, which has a Cd of something like 0.27:
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