Try codebender.cc: "online development & collaboration platform for Arduino users, makers and engineers""Code: Awesome editor, fast cloud-based compiler, helpful error reporting. Code faster and easier"Deploy: Connect your Arduino via USB, or program your Arduino Ethernet remotely over the network."Share: Share your work, search for existing projects and clone them, or collaborate with others.I have not tried it myself but it's probably your best bet if you want to do Arduino development with only a smartphone, tablet, or chromebook.
I know this thread is a little old but just in case others are looking for this answer I thought I'd comment. I do all my arduino coding on my chromebook and can even upload sketches. To do this I use a script called crouton to install linux alongside ChomeOS so anytime I need to do something ChromeOS can't do I switch over with a short keystroke and open that application. It's very smooth and so far has been working great.