The PMBOK Guide has never been a methodology, per se. It is an American National Standard and soon to be an international standard for project management. According to the introduction to the PMBOK Guide Fourth Edition, its purpose is:
- identify a subset of project management knowledge that represents good practice (though some might feel with hundreds of volunteer project managers participating, it is best practice)
- promote a common vocabulary
- be a foundational reference
- guide practitioners
In its most general definition, a methodology is about how you are going to do something. So at least at a high level, the PMBOK Guide is also a methodology. The PMBOK Guide outlines 5 processes and 12 knowledge areas and I have used it to build methodologies and augment existing methodologies. Drawing from the good/best practices, I have outlined steps to take in managing projects from a product, process, and people view, provided guidance and training for each step, and tools and templates to support each step.