I realized (thanks to zabdiel's comment on my first post) that I hadn't actually posted on what my first Android app was going to be (assuming you don't count my funky live wallpaper: maybe I'll put that up here some day, might be useful to someone).
It's intended for train travellers (but could have other uses). It's intended to be the electronic equivalent of hanging a sign round your neck saying "Please wake me when we get to Nuneaton", but getting your phone to wake you with an audible (or otherwise) alarm, using its GPS or other location services.
There appear to be apps like this for the iPhone, but I'm deliberately not investigating too much: I'm going to do it anyway as an exercise in learning Android developing, and if it's very different to the other available apps it might stand more of a chance of getting noticed.
Code can be found here:
Current state of the app is that you can click a button, and it pops up a map with your current location marked with a (rather too large) X. Still very early days!