I've finally started to read up on GLSL. It's nicer than I expected, I wish I had looked into this years ago. It's surprisingly easy to do some pretty cool effects - at least in a simple standalone test program. I am starting to realise, however, that fully integrating shaders into a game is quite a lot of work. And unfortunately I'm having trouble comitting much effort into my engine these days.
Anyway, here's a screenshot of a simple shader to compare basic lighting in the fixed pipeline with shaders. (The sphere on the right uses a shader, in case it wasn't obvious).