In previous versions of Outlook, most forms had a field on the lower right called Contacts which you could use to link a contact to an item which would then show up on the Activities tab. You either used this feature, or you didn't and for those who use it, Outlook 2007 hides it by default.

To enable it, go to Tools, Options, Contacts Options button and add a check to the option under Contact linking to show it on forms.