A very comprehensive article in Smashing Magazine - Email Newsletter Design: Guidelines And Examples discussing the various user interface and CRM methods used in email newsletter marketing. The article also includes a good range of examples showing various email newsletter design styles.
As you would expect - Apple's email newsletter for the iPhone 3GS is a mixture of clean stylish design and informative content.
The article however doesn't go into the technical details of how to best build a newsletter (such as the above example.) This raised an interesting point as to whether image heavy newsletters were acceptable to use - provided you assume subscribers want to read your newsletter message and would enable images in their email client.