So, is diffraction an issue in car audio as well? I'm thinking both of front systems and sub boxes in particular.

Its probably more apparent in Car audio than anywhere else, to be honest. Smaller space, you'll notice it more.

Most general users won't know what it is/hear it though.

So............................... what is it? What does it sound like?

In this case it has to do with sound-waves and how they react when bending around (the baffle) corners.

http://en.wikipedia.org/wiki/Diffraction

I didn't expect it to be of importance, but since it apparently is, I will take the precautions I can when I get there.