Have you looked at the Atrix Lapdock? It's an add-on for a Motorola phone that turned it from phone to essentially a laptop. It has an internal rechargeable battery, 11" screen, HDMI, USB, mouse, keyboard. Sounds like exactly what you want. Powers the Pi for at least 6 hours. Several people have used them. They go for around 70 quid on eBay at the moment, but they're cheaper in the US I think.
If you really want a bit of snazzy custom kit, check out the MoPi board. It takes pretty much any power input and converts it for the Pi. 8 to 10 hours on 8 AAs. With a screen and Wi-fi that time will go down, but should still be pretty respectable. What's pretty neat in this case is the ability to hot swap battery packs. Current batteries getting low? Replace the pack on the go! I loved it when my old Dell Inspiron 8100 did this, but with most laptops being much smaller and focussed on reducing weight, it's pretty much a dead feature. Oh well. The MoPi board also tells the Pi when the batteries are low so that it either shuts down nicely or prompts for new batteries.