Fluid grids, flexible images, and media queries are the three technical ingredients for responsive web design, but it also requires a different way of thinking. Rather than quarantining our content into disparate, device-specific experiences, we can use media queries to progressively enhance our work within different viewing contexts. That’s not to say there isn’t a business case for separate sites geared toward specific devices; for example, if the user goals for your mobile site are more limited in scope than its desktop equivalent, then serving different content to each might be the best approach.
via A List Apart – Responsive Web Design. A List Apart on creating designs that respond to all users and devices.