Anyone watching the US healthcare debates over the last 6 months should have developed a strong appreciation for the Congressional Budget Office. At different points in the summer, the two sides were each on the defensive as the CBO's costing of different initiatives either helped or hurt the case for reform.
What was interesting to me is that I read very little in the way of attacks on the credibility of the CBO during these debates. This NPR article on the credibility of the CBO suggests a lot of "screaming" on both side of the aisle, but in my view it is more whinging than strong criticism--attacking the CBO was not a central part of either party's strategy. Both sides more or less took their lumps when it came to the CBO. And remember here, this is not a polity that is averse to character assassinations and attempted credibility-destruction.
This credibility was born of legislated independence and nurtured by a history of solid research and good leadership. Clearly, the PBO isn't there yet. But I think the CBO is indeed a nice target for what an effective PBO might offer.
But don't take my word for it. Just check out what they do here. There is even a blog. (Can you imagine the reaction of the Library of Parliament and certain Parliamentarians if Kevin Page had a blog? The legislation does not mention a blog! The legislation! Sedition!)