Responsive Design is the hottest topic on the web and without doubt the most important one for many years.
There are countless articles on the subject but if you want an overview or something to refer customers to, this recent article , 'A Measured Response', on the Intel Software Adrenalin site is worth looking at.
If you need to keep up with what's happening in the world of RWD the 'Responsive Design Weekly' email newsletter is well worth signing up for. The site also has back issues and is worth digging into for tips and inspiration.