Interesting to have this happen now; I have no idea whether the Nobel committee cares about Krugman's public intellectual role. He's obviously a meritorious pick in any event for his economics work, which reflected a trajectory from (a) challenging the conventional wisdom, such as about international trade, to show that things were a bit more complicated and ambiguous, to (b) defending the conventional wisdom against rejections of it that were ignorant and overstated.
A word on his life as a public intellectual, although it's unrelated to the Prize as such. While I have confidence in my own judgments and feel that they usually end up being vindicated, he certainly was right and I was wrong about Bush, say, in 2000 - he got there several years before I did, at a point when I thought that, despite the wacky tax and budget proposals, Bush would be reasonably safe and not that different from his dad. At the time I thought he was too selectively harsh against one side. This reflected that I didn't realize the full enormity of things, or how different it was from politics as usual, for several years after that. So unless it was just a lucky strike, Krugman truly deserves the Cassandra Prize as well (since Cassandra was disbelieved but right)
Some conservative economists I know will be gnashing their teeth. Not, however, Greg Mankiw, who offers a couple of links describing the economics work and criticizing (though reasonably and interestingly) the op-ed columns.