The study was published in the journal, Psychological Science and summarized in a New York Times article.
The researchers had one group brainstorm in a neat room and a similar group brainstorm in a messy room. The outcome:
"When we analyzed the responses, we found that the subjects in both types of rooms came up with about the same number of ideas, which meant they put about the same effort into the task. Nonetheless, the messy room subjects were more creative, as we expected. Not only were their ideas 28 percent more creative on average, but when we analyzed the ideas that judges scored as “highly creative,” we found a remarkable boost from being in the messy room — these subjects came up with almost five times the number of highly creative responses as did their tidy-room counterparts."
They say a similar study at Northwestern University showed that people in a messy environment drew more creative pictures.
On this basis, I should be one of the most creative people in the world. Now if I could only find my desk...