Forbes has a great article on The Trouble with Responsive Design. The content of the article is less dramatic than the title and explains when responsive design might or might not be used.
One thing the article doesn’t cover is the case of using Agile with responsive design. Responsive web designs, or more specifically the implementations, quickly get complex and this, together with the relative immaturity of implemetation tools means that it’s often becomes difficult to refactor code during iterative development.
Trip Wire Magazine has a great list of responsive web design tools. As you can see, most are for either design or initial web site template generation. Once you go beyond simple layouts (’publisher’ sites as the above Forbes article calls them), it gets tricky to change the implementation quickly. This is especially so when, as the article explains, trying to present more unique and innovative experiences to mobile customers.