I ordered a book from iPhoto 5 a while back using one of the softcover books and was less than impressed with the print quality; this was before discovering that iPhoto 5-generated PDFs were defaulting to a lower DPI than what your original photo data contained.
There was a tip to increase the DPI in iPhoto 5 by editing a plist value and I remember reading that iPhoto 6 upped the default.
Using a utility like Pref Setter to browse the "~/Library/Preferences/com.apple.iPhoto.plist" file, look for the "BookTargetImageDPI" and following book settings to see what your defaults are set at (mine read 300 DPI).