I think its more plausible than alien messages, ley lines or any of the other ridiculous theories for crop circles. Plus there is no evidence of anything else making them, the one supposed video of CCs being made by aliens was later admitted to be hoaxed by the guy who did it:

http://skeptoid.com/episodes/4062

I already referenced this article in my previous post, but here is the video in question:

http://www.youtube.com/watch?v=jMeRd5EdBwE

The whole documentary:

http://www.youtube.com/watch?v=wW3Ls...eature=related (Part 1)
http://www.youtube.com/watch?v=XFTYP...eature=related (Part 2)
http://www.youtube.com/watch?v=MChU9...eature=related (Part 3)
http://www.youtube.com/watch?v=Nu6V0...eature=related (Part 4)
http://www.youtube.com/watch?v=D5R4E...eature=related (Part 5)