(As some others have suggested--) I'm moving on from wireframing except when I can draw it by hand. Prototyping with tools like Twitter Bootstrap provides infinitely more value to the client and addresses most of your points (esp re exportability and responsiveness). It really doesn't take much longer (if any longer) than producing a wireframe. If it is, you're either doing it wrong or should brush up on your html markup skills. The need for your own CSS as you suggest to one response is minimal.
