Our editing surface will be the Word, regardless of what's installed. If the Word12 product is installed in additon to Outlook12, the you'll get the full Word editing experience in Outlook email.
If Word12 is not installed, then some functionality won't be available when composing email, but you'll still have more functionality than was available using "trident" in Office2003.
Also, in 12, Word will always be the rendering surface for email you've received, even in the Reading Pane.
The Word team has worked hard to make sure we have high fidelity for rendering and composing. It's been truly amazing to see this great work.
One more nugget: Outlook12 now uses Word as its editor for Outlook Meeting Requests, Appointments, Contacts, etc.