Microsoft changed the rendering engine in Outlook 2007 to Word's HTML-engine, and it really sucks concerning modern design habits (css-positioning, etc.). So you have to look out for some pitfalls. I found a nice guide for designing HTML-Mails for use in Outlook 2007, which can be found at http://www.duoconsulting.com/downloads/contribute/SevenDesignTipsforOutlook2007.pdf.

Also, Microsoft offers tools to validate HTML files for the Word rendering engine. These can be found at http://www.microsoft.com/downloads/details.aspx?familyid=0b764c08-0f86-431e-8bd5-ef0e9ce26a3a.

So, good luck with your e-mail marketing ;-)