Mr. Ai is well known in China due to his designing of the Olympic Bird's Nest Stadium. He is the son of prominent 20th century literary figure, Ai Qing. Presently Ai Weiwei has a favourably reviewed exhibit at the Tate Gallery.
His arrest can be seen as a milestone in the Chinese authorities' suppression of dissent due to Mr. Ai's prominence and very good reputation among most Chinese people in China.
Insightful article about Mr. Ai in New York Times: http://www.nytimes.com/2011/04/06/arts/design/ai-weiwei-takes-role-of-chinas-conscience.html?_r=1&pagewanted=print
Very powerful video statement by Mr. Ai: