How do I add conditional style in rails 3.1

Hello,I’m using blueprint and the css looks like this:

for rails 3.1, it has asset pipleline ,compiles all the css into one
css file,
so how do I handle conditional including stylesheet like <!–[if lt IE
and how do I handle proper different media like that?
(besides, each css file has some additionaly meta like @charset “utf-8”;
when it compiles together, does those files conflict with each other?