Even the lightest notebooks seem to struggle approaching 4 hours of battery life, despite numerous promises from the industry to give us all-day (8 hour) machines. While we see a lot of promising technologies emerging to boost that, such as Flash SSDs, it's still a ways away. To that end, we are getting close and AMD is now promising that Turion notebooks this year will be able to boast a 5 hour battery life. Their extensive focus on mobiles, probably largely in part to catch up to the successful revisions to Centrino, has led to new low-power chipsets combined with more efficient CPUs:

At the heart of the 2007 Kite refresh is the RS690T chipset -- a low-power version of the RS690 chipset found on the desktop. Graphics are provided by a Radeon X700 derivative core; system IO functionality revolves around the SB700 southbridge.
That'll also be coupled with 65nm Turions, giving a significant reduction in power from the current 90nm chips. On some models of machines, that boost will give an extra hour of battery life. The new Turion platforms also rely on the more power-efficient DDR2, and perhaps coupled with SSDs and low-power GPU cores we may see that 5 hours get closer to 6. Definitely worth paying attention to.