I'd suggest you have a look at media-queries. This is a useful new addition to CSS that really makes so much sense, you'd wonder why this wasn't implemented waaay earlier. Basically it allows you to target browser attributes (such as max and min-widths) directly via CSS and branch your layout code from there. Similar to creating a print stylesheet, you can create your desktop and mobile layouts in parallel in the same file, which kicks ass for development.
