I use an iBook G4 and it is awesome for battery life and heat. 3 hours is almost a minimum battery life, 4 hours is common, and 5 hours is quite realistic for light use. The warmest component is the hard drive and the system fan doesn't turn on unless there are lots of CPU-intensive processes running. I mainly use my system for web browsing, email, music listening, and word processing. Sometimes I'll use it for more demanding tasks like audio and video conversion, but it's more than adequate for that. I just installed Ubuntu on it yesterday since I needed a greater app selection and wanted a more customizable system, but ran OS X before that since last July when I got the machine.
If I were getting an x86 laptop I'd likely get an Intel Core Solo since it's about the most power-efficient x86 chip out there from what I understand. If I were to trade off some battery life for performance, I'd get something with an AMD Turion. AMD chips have an awesome price/performance ratio and they tend to kick Intel CPU's butts in per-clock performance. My desktop system is an old AMD Athlon XP Barton system and it is still quite fast even nearly 3 years after I built it.
At least nowadays there are lots of choices for the $1000 and less laptop market.