It depends on what you call "commercial purposes"...
I believe that many people earned money by making shows or installations or videos based on Processing. A look at the Exhibition section of the site should show a good number of them. A famous example is a Radiohead video clip made with Processing (and lot of costly hardware, etc.!).

I am not aware of a pay software built upon Processing, thought. To my knowledge (IANAL), nothing prevents this: Processing is GPL, but code produced with it can have any license (AFAIK) including commercial ones, as long as you use the binaries as provided. If you alter the core.jar, you have to release the modified sources.

http://vimeo.com/flight404/videos    This is hard to ignore. It s a good argument of some one that made a living out of Processing. The more recent things this dude has done has taken a direction more towards Pixar CGI etc C++ Python. but the old stuff was Processing and there is a lot of stuff meshed together here.