How often is the PMBOK Guide revised?

The Project Management Institute (PMI) revises the PMBOK Guide every 4 years or so. For example, the second edition of the guide was released in year 2000, the third in 2004, and the fourth at the end of 2008 (beginning of 2009).

With this trend, you can expect the fifth edition of the PMBOK Guide in 2012-13.