Designer Elliot Jay Stocks on responsive web design (RWD):
RWD is about making your site adaptable to any scenario, without worrying about specific devices and their proprietary dimensions.
RWD doesn’t need to take more time and therefore doesn’t need to cost your clients more money. At least not after you’ve rethought your approach to web design, anyway.
His argument isn’t perfect – I think Elliot underrates costs overall, but it’s still very strong.