Following other theme providers as well as Drupal core, I'm dropping support for IE7.
The new branch of the Arctica base theme is now using a more lightweight grid system that uses the border-box model. While it is possible to polyfill IE7 with border-box support I soon found out it doesn't work reliably. Instead of spending time on trying to bring old browsers to the future I'm now going to focus on leaner code and better features. IE7 simply can't cope in a world with responsive web design and HTML5 features that are gaining support in modern browsers.
Hopefully IE8 will soon follow: http://theie8countdown.com/