Screen sizes are getting smaller again
With more low-resolution devices being released all the time, such as ultra-mobile and ultra-cheap laptops (eg the Asus Eee PC), handhelds with mobile/wifi connectivity (such as the iPhone and the PSP), it is rapidly becoming unsafe to dismiss certain screen sizes when you redesign a website. QVGA (320×240) is the dominant screen size for mobile phones, WVGA (800×480) is increasingly common for ultra-mobile devices.