Pluto's status as a planet was "revoked" today by the International Astronomical Union. Our solar system contains millions of objects in a continuum of sizes and shapes but in our quest for simplicity we have chosen a model of the solar system based on one star and a few planets. As with any definition there are borderline cases and Pluto has been determined to fall short of what constitutes a "real" planet.
Lots of books and other information sources shall now have to be revised to reflect the new world-view. As you would expect, the process is already in progress on Wikipedia where a veritable edit war has broken out. In the first hour after the IAU's vote, some 50 changes were made to update the article on Pluto. It is impressive to see how quickly references to "planet" were replaced with dwarf planet and trans-Netptunian object and an explanation of the controversy was added. Soon after, somebody who must have felt strongly about keeping Pluto's planetary status tried to restore the article back to the state it had earlier in the day when Pluto was still the ninth planet. Several attempts at vandalising the page or inserting irrelevant information were corrected within minutes if not seconds.
Wikipedia has several times before shown its ability to react quickly to developments. You can watch revisions to articles as an animation using Wikipedia-animate.