Stefano has some interesting stats in his Screen Resolution and design... post. A large part of the increase in higher resolution monitors is due to the rapid increase is LCD and plasma displays being sold (which I think are now outselling CRT displays--good news for us all with small desks). Part of the LCD/plsma market is laptops, and Stefano's stats miss something important--screen size. Laptops have a much smaller screen size, and in a number of cases are the primary or only computer. Although designers can design for higher resolutions, it doesn't necessarily mean they can make larger pages.
My wife has one of those awesome little Sony Vaio TR-1A laptops. Amazing monitor, and it has the dimension ratio of an HDTV. But it's the size of a paperback book--such a small screen size! Navigation can be a pain on sites that were built for 17” monitors at 1024x768.