After much experimentation, when I upgrade all my kit, I'll be buying a small, powerful notebook with wi-fi, all the usual whistles and bells and a very good screen: something like this, plus a seperate Hard-drive, 250Gb-ish.
Sorry can't form a link - look on the Sony Website for the smallest notebook with the crystal black screen (just spent half an hour trawling through the most cr&ppy website ever - talk about complicated. If I didn't know it was the product I wanted already, this site would put me off buying).
After I take it out of the box, I'll delete all the branded Sony Vaio stuff and leave it running a basic Win XP-Pro platform with just the bare essentials of what I need - internet, Photoshop CS (though even Elements would be good enough for field work) and Word.