Did you get to the bottom of this? I have the exact same problem as I re-installed Windows 7 retail from Amazon this week and put Outlook 2007 on, ClearType is OFF system wide EXCEPT in Outlook message boxes, I can't see an option anywhere to remove it.
Edit: Figured it out, for some reason Outlook didn't want to take notice of the system wide setting, so once I had enabled and then RE disabled ClearType all was well.
Edit 2: Spoke too soon, now Office is doing what it liked again and is not respecting the system wide setting. *sigh*
Is there a way to FORCE office to obey what I am setting in Windows?