I just "ported" my OpenGL code to some Intel graphics chip (previously I've only tested on nVidia and ATI) and found a curious thing: face culling was implemented differently (on that particular driver?). It looked like the frustum decided which facets were front facing, rather than the projected modelview. I also had to redo the font rendering to use bitmaps, as raster fonts was too slow. (Always a lot of extra work when doing it oneself rather than using middleware, but then again I think it's fun!)
I beginning to wonder how many of these portability issues I'm going to run into before my first game is done.
