Bootstrap comes with 180 icons (glyphicons) in form of ttf, svg and woff files. Old one only has 140.
Old bootstrap used sprite image, so
'render' => 'sprite'
should probably change, but I don't know to what.
New bootstrap needs glyphicon class so I implemented this hook: bootstrap_preprocess_icon (why aren't preprocess functions called from included files? I had to put it template.php to get it working...). I don't know if it's the proper way to do that.