From the website:
Ubuntu 9.04 Desktop Edition delivers a range of feature enhancements to improve the user experience. Shorter boot speeds, some as short as 25 seconds, ensure faster access to a full computing environment on most desktop, laptop and netbook models. Enhanced suspend-and-resume features also give users more time between charges along with immediate access after hibernation. Intelligent switching between Wi-Fi and 3G environments has been broadened to support more wireless devices and 3G cards, resulting in a smoother experience for most users.