Microsoft has confirmed they plan on using the Word rendering engine to display HTML emails in Outlook 2010. This means for the next 5 years your email designs will need tables for layout, have no support for CSS like float and position, no background images and lots more. Want proof? Here's the same email in Outlook 2000 & 2010.
Microsoft has developed a response defending their plan, and FixOutlook.org has a has also shot back"