I do most of my arduino and processing development on a tiny eee pc.
The key thing that makes this easy is a compact programming language I think. Plus I'm usually making embedded software so it's small and cheap enough to leave lieing around running sensor logging scripts etc.
Another commenter said:
it's pretty rare that I'm out somewhere and wish I could develop something but haven't brought my laptop
Perhaps hacking embedded "internet of things" type software where there world around you is a rich hackable environment is what will drive handheld development machines.