A caller asked this question, noting that polarized sunglasses seemed to create or enhance this pattern. (I have found this to be the case as well.)
Bill responded that the pattern was an interference pattern caused by the polarized sunglasses, although I don’t believe this is actually the case.
Having wondered about this myself, and noting some other aspects about the visibility of the pattern (why do I never see it on the windshield, rarely on the side windows, and almost always on the rear window (also called the backlight), I asked the same question of a fellow involved in automobile design. He believed that the pattern appeared in tempered glass, as a result of the manufacturing process. Tempered glass is designed to shatter into very small (smaller than pea-sized) chunks without creating sharp shards of glass. This is accomplished by rapidly cooling both sides of the hot sheet of glass with cold air streams. Because the outside cools much more quickly than the inside, a stress pattern is created in the glass that causes the glass to break in the desired manner (which you may have seen at the site of car accidents, and next to broken-into cars in the bad parts of town…).
Car windshields don’t use tempered glass, but rather laminated safety glass, in which multiple layers of glass are sandwiched with layers of plastic (or vice versa). This keeps the entire windshield in one piece (in most cases of impact), to keep the driver from having to deal with that amount of broken glass coming his way in an accident, or as a result of dings in the glass caused by pebbles.
While I believe side windows use tempered glass, they may be too thin to exhibit the polka-dot pattern in most lighting conditions.
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