WP Themes by Screensize will allow you to determine which of two wordpress themes to display to the site visitor depending on their browser width
I didn't want to be checking every new page to see what the screenwidth was. On the other hand, I wanted to be able to accomodate changes in browser size whiole someone was viewing the site. Setting the cookie duration will set the interval between checks. If it is set to say 60 seconds, the browser width will be checked on the first new page or page refresh after that time.