I initially got an SPUrl error when using the code in the book (page 100 on my Kindle). I had imported the NavDemo.master from the downloads. I did some digging and used the following code instead, which solved the error.
<SharePoint:CssRegistration name="/Style%20Library/SPDDemo/css/roundednav.css %" After="corev4.css" runat="server"/>
However, after copying the images, etc and importing the roundednav.css file, my browser does not look like Figure 3.27 on page 104. (Incidentally, I copied all of the images, etc into my main site and the sub-site).
I do not see any tabs at all in my browser.