Well I just bought an 8600, because it has Centrino technology. It uses the Pentium M processor. You don't sacrifice any speed by going with the Pentium M, and you gain a laptop that is lighter, (nearly 2 pounds in this case) runs cooler, and whose battery lasts longer.
If you value Battery life, you have to go with something that has Centrino.
However, there are lighter Centrino notebooks out there (the Compaq X1000 for example, is 0.4 lb lighter). Ken says the case is weak, but I can't comment on this as i haven't received mine yet.
I've been trying to decide between two Dell notebooks, the 8600 or 5160. One of my consideration is battery life which I have not been able to find a good article on. I've read pros and cons on their tech support, so I'm not sure how it rates to other brands. 've been able to configure both under 2000 which included the warranty option. Would appreciate any advise or a suggestion on another brand.