I bought a battery pack from Proporta, it's Li-ion and has 3400mAH capacity. Unfortunately, it arrived a day to late for me to take on my last trip abroad but it's kept my N800 running for about 8hrs while streaming Internet radio over WiFi. That's abt 5 or 6 times what I get with the internal battery.

I'd thought of building my own using AA NiMH cells but the Proporta part was better value & more likely not to cause hassle at some airport security check.