If you are seeing Copy: [subject] on appointments you copy between calendars and wonder why it’s happening, it’s a new feature of Outlook 2007 SP2. All meetings copied from other calendars are now identified as copies. The release notes for SP2 say Copy: is added to meetings copied from shared calendars, but in my experience, it happens on meetings copied from any calendar, including secondary calendar folders in your mailbox.

No, you can’t disable it. It serves as a visible reminder the meeting was copied from another calendar so you know to look on the original for updates.