Microsoft takes email design back 5 years. Outlook 2007 uses Word 2007's HTML rendering engine, not IE7's, meaning that a fair amount of HTML and CSS that rendered properly before will no longer work. The post I've linked to points to the relevant MSDN articles with more details.

While I'm not a fan of HTML email, I realize that it is very important to emarketers. We have a few web-based newsletters that we send out at work, so I guess I'll need to plan to audit the code and see if any adjustments will be necessary.