Chinese Human Rights Defenders reported yesterday that Xu Zhiyong was formally arrested on August 18, 2009 at 11:50 AM.
Holding a person in detention for many days prior to arrest is not uncommon. While the norm is three days before a request for an arrest, in political cases it is not uncommon to hold an individual for up to 30 days before a formal arrest. For a great summary on the multiple ways that a person can be held by the police without an arrest in China, see this post on China Law Prof Blog.
The charge against Xu Zhiyong – tax evasion.