I saw a show on the discovery channel, or TLC, that show a couple groups of people who did it. They concidered it an art form, and were really good at it. They even showed a time-laps video of the whole prossess.
Ok, I think it's important to note that while many of the recent ones have been attributed to "makers", some of the older ones have been ruled as NOT being "hoaxes". I think the TLC/Discovery special even implies this in the beginning of the segment.
I've seen documentaries that show people making them, but they're not nearly as complex as some of the ones popping out in the last couple years. I swear, I think skeptics would have second thoughts at some of ones popping out recently. Not just an emsemble of shapes, but patterns that could actually be interpreted as well. I think there is a decent pic gallery at www.artbell.com somewhere.