I've been dabbling with a Drupal responsive theme and have seen a few people I respect (and I think that you do too - incredibill and coopster I think) strongly recommend Twitter Bootstrap for responsive HTML5 designs.
Anyway, I haven't tested on a wide range of platforms, but I'll just throw out that if you're going to do responsive image resizing or otherwise push the limits, you can expect some conflicts with, say, jQuery image manipulation for image sliders or overlays. Things like that have to be a bit more carefully constructed than you might be used to.