What I imagine will be most annoying about programming on a mobile device is the amount of scrolling needed to move around the code. Sometimes I find my 17" laptop screen too small, so a phone seems unusably small.
Back in the day, 320x200 - 40 columns x 25 rows - Commodore 64 seemed OK, until I saw the hires mode on an Amstrad CPC 64 - with 640x200, giving 80 columns. All these were huge luxuries compared to the 20x1 LCD character screen I mention in another comment here, of course.
But the point is, the problems you'd tackle are smaller too. Just ad-hoc things: probably simple mashups using maps, GPS and connectivity; algorithm implementation to satisfy curiosity, or to learn in idle time, or even to brute force a puzzler in a newspaper column. There was an iPhone game, Fling, which I spent 20 minutes writing up a solver for, on my laptop. I'd have liked to do it on the phone instead.
Why would one want to code on an Android device? Not everyone uses laptops all the time. Laptops need to be lugged around (even though mine's a Toshiba Portege, and is lighter than most at ~1.2kg, I seldom bring it anywhere), opened up, resumed (takes a second or two), balanced on a knee (so you need to be sitting), etc. Hard to do when you're e.g. waiting in a queue, walking in the countryside, lying in bed (without rearranging pillows, sitting up, lighting up the room from the screen, fans and light disturbing your significant other, etc.)
And then there's the idea that you're writing programs for your device without the need for a deployment stage. You're running them right there on the device. No need to faff about with cables, or build / deploy steps.
As to lines of code, in a structure-oriented editor, I think that would not be relevant. I would imagine program navigation to be spatial with drill-down, up and out; all identifiers are hotlinks to their definitions; MRU lists and the same Patricia trie technique, or incremental search, for more random access, etc.