Outlook 2007 has been created using the Word rendering engine unlike its previous versions that made use of Internet Explorer's one.
What this means unfortunatly is that Outlook 2007 no longer has a very solid CSS rendering capability. The main problem that this is causeing so far is that Outlook 2007 does not render background images at all. This means that Most, if not all, of Interspire Email Marketer's built in templates prior to version 1.3.1 are made obsolete.
For a look at Microsofts listing of what elements are and are not supported by Outlook 2007 please have a look here:

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