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Yeah, I know about this phenomenon.  They are also called hole-punch clouds or fallstreak clouds. On the link you provided, the author claims that they are rare. In fact, they are pretty common, especially in the winter time. They form when a section of a cloud deck rapidly condenses, since it is composed of super-cooled water droplets, and falls out as precipitation (most cases it is snow). Since the atmosphere is dry, the snow evaporates before it hits the ground. So, you see a pretty uniform cloud deck, with a seemingly circular hole in it with a feathery portion streaking downward in the middle.

 Passing airplanes can induce hole punch clouds, and they can form long canal like gaps in the cloud. Check out this image:

http://en.wikipedia.org/wiki/File:Texas_tmo_2007029_lrg.jpg

I've actually seen these myself a number of times, and I took some nice pictures of them. Though, I can't remember what I did with them.