I'm looking for a portable tablet pc which I can do some drawing but mostly painting.

Though I do like to take a sketch pad out with me and draw traditionally, I take long train journeys quite frequently which can be up to six hours just one way so twelve hours round journey but I'd still like to be able to paint.

I have tried taking my laptop out and using my intous 4 but you don't always have a table seat or that much room to actually have every out at once and it is quite cumbersome. I have looked into the ASUS EP121 but the battery life on it just isn't long enough.

Does anyone know of any tablet pc's that would be suitable? My price range is up to 1000

Requirements:

Portable
Responsive
Runs Photoshop CS5 or above
can maintain a long battery life (while using Photoshop)