Humans?
Some of the circles are so huge, it is a staggering thought to even think some croppies could do this without damging the plants, or even getting caught. Trust me, there are a lot of loyal "croppies" watching for the seminal event that would LOVE to catch the hoaxers. And so would the farmers.
Engineers have been hired to try and re-create some of the designs in the fields, and asked about their labor and time frame. NONE could produce what is done overnight or within minutes, as some of these circles go down. Mind you, I also admit that most of the simpler ones are man made.
Here's the original article for those who missed it the first http://www.earthfiles.com/news.php?ID=1434&category=Environment