With CDs and DVDs on Mac OS X, the /dev/diskN entry seems to be the raw disc contents (i.e. it includes things like media error correction data). For imaging, you want /dev/diskNs0 (or sometimes /dev/diskNs1 etc, especially on a multisession disc). If the disc has a partition table, the mountable volumes will actually be e.g. /dev/diskNs1s2.
BTW, you can also create iso-style images from the GUI with Disk Utility: select the session from Disk Utility's sidebar (note: the session will be the item directly under the disc drive in the sidebar; if the disc has a partition table, there volume will be listed separately under that, and it's not what you want), then go to the menu and select File > New > Image from . Then, select CD/DVD Master for the image format. It'l name the image with a .cdr extension, but the format is the same as a typical .iso image.