Dias Ex Machina
Publisher / Game Designer
Early in the concept phase for ZEITGEIST I did consider including Pathfinder stats. We quickly realised, though, that you can't just substitute statistics (any more than we could for the 3.5 - 4E WotBS conversion) and that it would require a lot of rewriting for each system to do a job worth doing.
From personal experience (and take this with whatever grain is available) converting an adventure shouldn't be too hard. As for a campaign setting, it's vastly more difficult than one can expect. The entire design philosophy between 3rd and 4th is so radically different, there are elements you can get away with with 3rd Edition that you can't with 4th Edition. I have been accused of "following the 3rd Edition design philosophy"--a critisism that equates to not writing a 4E supplement to "feel" 4th Edition. Quotes from reviews have included them saying, "3rd edition is about saying 'no' while 4th edition was about saying 'yes'" (I'm not agreeing; it's a quote). I have written Amethyst for both 3rd Edition AND 4th Edition. The games act and feel very different. 4th edition, by my observations, is more difficult to write for, as it is more strict with wording and balance.
I am only making this statement based on my experience with 3.5. If Pathfinder has moved into the same philosophy, then I recant my statement and state that it would be hard, but not as hard as it was adapting 3.5 to 4.0.