Yes, I meant "guessing" the right size based on the user-agent string as the best approach (since it doesn't rely on the browser doing the work, it will work with any browser, including old ones).

But I also maintain that a single image of relatively small size can be used for all form factors if its quality is good enough (resized by the browser based on CSS instructions); you can at least double the size of a "good" image before you begin to see problems.