The idea of a monitor being certified for Vista is quite possibly one of the most pointless gimmicks I think I've seen in a long time.
Vista will work on any monitor capable of, well, displaying a picture at a certain minimum resolution - any monitor in the last 12yrs can do that. It doesn't have to certified, it just has to work.
Apparently (and I've lifted this off a BenQ website somewhere so this is heavily active selling here):
LCD monitor market influenced by Windows Vista
Accelerated demand of LCD Display
Windows Vista creates great demand of system upgrades from computer users. Customers will look for LCD monitors certified by Windows Vista.
Widescreen becomes mainstream
Windows Vista is designed to support wide-screen monitor displays for enhanced multimedia features. With bigger size wide screen display, users can be benefited by new features such as Windows Sidebar and gadgets.
Windows Vista's core entertainment features, such as TV, movie, and gaming will result users to opt for larger sized displays. 19", 20" and 22" will be the main choices when consumers consider system upgrade for Windows Vista.
Growing Demand in HDMI Interface
Windows Vista interface elements require high quality display and entertainment features are emphasized. Therefore customers are keen on high quality display solutions. It's expected that monitors equipped with HDMI interface will become more popular in 2007.