The PMBOK Guide is an ANSI standard. ANSI audits standards developers on a 5 year cycle and I've found most are updated within four years. PMI, I believe, intends to follow a 4-year cycle. If the last update was 2008, then the next would be 2012. Possibly coming soon: and ISO standard PMBOK Guide derivative. Generally a document must be an ANSI standard before moving on to be an ISO standard. This will put the PMBOK Guide out as a global standard.