This is not a support question, as I do not own eleven40, but figured you would want to know about it, as it's rather glaring.
check out the screenshot from my laptop:
Notice the left sidebar is below the content area instead of on the left side. This is because you are using fixed widths in the theme and the widths can't handle the total before the media query breakpoint is reached. Honestly you should be using % instead of fixed widths on this theme here.
To replicate, simply choose the 3 column layout and resize your browser window. It will happen on every browser. As we can't count on a users browser width with so many devices on the market today, this is a rather large problem for a responsive theme to have.
I found it because I was searching for a good responsive theme to use for a project and I see a lot of people making this same mistake. There are a lot of themes on the market that do not have this problem. I can send links if needed to see how to fix it. I just figured as perfectionist as Genesis is, you guys would want to know about the problem.