It seems everytime I drive on the highway, I can smell every single diesel exhaust fume that comes into my car...

What gives?

closed you shouldn't be able to smell anything.

I always leave the climate control set to Auto, with fresh air (not recycled) and I rarely smell exhaust, when I do it's only a quick whiff. My previous car (2002 A4) and my current car (2005.5 S4) both have cabin filters. Only when I get stuck right behind a bus do I really notice it.

I just had the 35K maintenance done, and in the details it says:
4B0-819-439-C FILTERELEM (Fresh Air Filter)

I'll ask in my next service visit..