The battery life sales point is an interesting differentiator. By my quick survey it looks like most laptops get about 4-5 hours of battery life now a days.
The non-user-replaceable battery seems to be a gamble that hasn't hurt them in the market.
Some ultra low voltage processor models get around 10hrs of battery life as well. The Asus UL series get really good battery life, at least under Windows.
I have a UL80vt running Ubuntu 9.10 and get around 5hrs on that with the screen at full brightness (b/c brightness control only works with reboot). It also uses the nvidia dedicated graphics afaik (it has hot-switchable dual graphics under windows).
The non-user-replaceable battery seems to be a gamble that hasn't hurt them in the market.