The draft release of the PMBOK 4th edition is out at the PMI website for review. I spent some time going through it yesterday and there aren't any radical changes. If you want to provide your input, you have until March 22.
They have removed some of the management plans like scope, cost, and time. They got consistent in naming the processes, they're now all verb-object (e.g., Define Scope). They've also simplified some of the inputs and outputs to processes. There are now sections for gathering requirements and identifying stakeholders. The procurement area got the biggest overhaul, it's now down to 4 processes from the 6 there used to be.
There's still no mention of agile or lean or any related techniques. I have seen a trend of more of us traditional (i.e., PMP certified) project managers adapting agile techniques so I'm disappointed this wasn't included in the new PMBOK.
In general, it's evolutionary rather than revolutionary. I guess you can look at the updates to the PMBOK as a iterative approach, only the iterations are 4 years rather than 4 weeks like a Scrum project.