My office is moving to Office 2010, and I need to set the default font. I have a custom Normal.dotm that works in Word, but I have not found a way to force the default font in Outlook 2010.
Outlook stores it's default font settings in the registry.
*These instructions are for Outlook 2007, but should be (basically) the same (I don't have 2010 on hand to verify):
Go into Outlook (ensure regedit is closed), Tools->Options->Mail Format->Stationary and Fonts. Setup the fonts the way you want them.
Open regedit, go to the key above, which should now be updated with the font information you set, and Export it.
Take that exported .REG file and import it into the machines (either manually, or perhaps by login script and Reg.exe, or under a 2008 domain you should be able to use Group Policy Preferences to make the registry changes).
Hope that helps...