Ok, to explain as simply as possible: You cannot watch a copy-protected conventional DVD (95% of the DVD's sold) with Vista via a digital-out (ie, DVI) that has a resolution higher than 720p. Since virtually any big-OEM system that will be sold as "Vista Ready" will be using DVI and LCD monitors that are higher than 720, there will be plenty of confusion about why people's WinXP systems would let them watch DVD's, but their "new-and-improved" Vista system gives them a "Windows Media Player cannot play the DVD because a problem occurred with digital copyright protection." error.
Don't believe me? Just go ahead and try it..I'll wait.
This topic has been discussed to death on the Vista forums...try doing a little research before anonymously posting about topics you don't understand.