If e-mail marketing was a stock, I would hedge it. As I told Marketing Mag:
While its effectiveness will continue to decline in 2010, it will still provide one of the highest ROIs of any marketing channel. That said, I believe it will be incrementally harder to persuade consumers to sign up for e-mail campaigns. E-mail will also need to be integrated into other marketing channels such as social media. We need to figure out how the content we create is truly distributed and how many times our targets see our messages.
Further to this, one of the main reasons I am less keen on e-mail marketing is the growth and proliferation of smart phones, or mobile devices. On the BlackBerry in particular, e-mail marketers and, more importantly, vendors such as ThinData and Lyris (formerly EmailLabs) need to solve is the usability issue of a smart phone receiving an html-heavy email, as I did today:
I love the Bulldog Reporter’s “Daily Dog” email but this delivery to my BlackBerry makes the great content almost unusable – especially at 7:12 am!
If this basic user experience issue can be solved, we can move on to solving the email overload issue – the real issue facing e-mail marketing.
Ed Lee's Blogging Me Blogging You | @edlee 
The basic user experience issue has been solved: stick with straight-text emails.
You never know what sort of device or system the person on the end is going to be opening it on, so go with the lowest-common denominator.
While the emails won’t be able to be as flashy, you can still get your message across with a well-written piece.
I think Dreamhost and the HypeMachine have been particularly good at sending marketing or update emails that make good use of plain text.