Before I buy any books for 4.0, I would like to know if the physical books (PHB, DMG, MM) have been edited after WotC updated their online definitions. This has been bugging me for a while now. A friend of mine said he bought a book this year, but that some spell definitions and such were not updated to the definitions that were released some years ago.
The books are not updated; if you buy them, you are expected take care of the updates yourself, by perusing the official rule update PDFs.
The alternative used to be simple: buy a subscription to D&D Insider. However, with the fifth edition coming soon, who knows how long they will keep the 4e sites running...