I write a lot of code on the OpenPandora which is about the size of a smartphone and runs a full version Linux (Debian in my case). Especially when travelling a lot I like it much better than carrying a laptop with all the cables around. I have been working on an environment to work productively on my smartphones but that proves to be a lot harder. Textual input is just not very nice so it needs something different which I think I figured out for some cases; general programming though I am not sure about. When I really have to, it is faster to just talk into whatsapp/wechat and send those snippets to my colleagues who turn that into code and debug it.

Hopefully this year I will have a Pyra (which actually can double as smartphone) and hopefully I will have progressed with my 'visual IDE'.

Edit: what surprises me is that not more people are working on this; we have more touch devices than anything and more (aspiring) programmers than ever; there is a market and a greater good there.