Though he has long been outspoken in challenging authority, no one knows who in the Chinese government detained Ai Weiwei, or why, or where he is being held. This is illegal under China's own law.
Here is Matthew Felix Sun from his blog of yesterday, asking "Where is Ai WeiWei?" (I've left out the Chinese bits):
Renowned Chinese artist Ai Weiwei, was detained by a certain Chinese agency (which one is not clear, because not a single organization in China has claimed responsibility or notified his family of his whereabouts) more than a month ago on 3 April 2011.
According to search results for Provisions of Criminal Detention Duration for Public Security Organizations yielded by the Baidu search engine, normal detention without formal charges may last from one to three days. For reasons such as preventing further crimes or escape, a special request granted by authorities of a township or above (such as municipality or province) will allow the detention without charge to be extended to 30 days.
Below is a translation from a document dated 27 January 2006:
Provisions of Criminal Detention Duration for Public Security Organizations
1. Released by Ministry of Public Security on 27 January 2006
2. The Public Through  No. 17
3. Shall come into force since 1 May 2006
Article VI. When a public security authority at the level of a township or above makes a decision to approve a detention, the accompanying report should indicate the length of the detention time of one to three days. If there is a need to extend the detention to one to four days, or to thirty days, follow the request to prolonged detention procedures.
Article IX. If it is not possible to ask the People's Procuratorate to examine and approve an arrest within three days after the suspects is detained, and if there is evidence that the criminal suspect may escape and repeatedly commit crimes, or commit gang crimes, the detention, after being reported to the public security authority at the level of a township or above, and if approved by the authorities, may be immediately extended to the thirty days.
Where is Ai Weiwei? China: the world is watching.