Well it turns out that I was mid-book when I asked this question, and the very next day I read the page where the author lists out his recommended breakpoints. Hopefully it's okay with everybody that I quote that passage here. If you're ready to jump headfirst into designing/coding responsively, Ethan Marcotte's Responsive Web Design is probably a must-read: http://www.abookapart.com/products/responsive-web-design

320 pixels - For small screen devices, like phones, held in portrait mode.
480 pixels - For small screen devices, like phones, held in landscape mode.
600 pixels - Smaller tablets, like the Amazon Kindle (600×800) and Barnes & Noble Nook (600×1024), held in portrait mode.
768 pixels - Ten-inch tablets like the iPad (768×1024) held in portrait mode.
1024 pixels - Tablets like the iPad (1024×768) held in landscape mode, as well as certain laptop, netbook, and desktop displays.
1200 pixels - For widescreen displays, primarily laptop and desktop browsers.