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Previous versions do it too. In 2003 and 2007, you can find these cached names in this hidden file:

C: \ Documents & Settings \ username \ Application Data \ Microsoft \ Outlook \ outlook.nk2

and in 2010:

C: \ Users \ username \ AppData \ Roaming \ Microsoft \ Outlook \ outlook.nks

2010 at least provides a way for the user to manage these via the new 'Suggested Contacts' folder.