In versions of Outlook prior to 2010, the autocomplete function stored the data in a file with a .nk2 extension. The file was kept in %appdata%MicrosoftOutlook. As it was a file, it often got forgotten when the user got a new computer or a new profile.
With Outlook 2010, this data is now held within a hidden folder either stored on Microsoft Exchange or in a PST file, depending on how Outlook is storing the data. This means that it is much easier to carry the autocomplete cache over when the user moves.
Windows IT Pro has a good article about the function and how it has changed in Outlook 2010.
What is really good to know, though, is that Outlook 2010 also has a function for importing the data in the .nk2 file. For complete instructions, read this thread, where Jennifer Zhan from Microsoft provides step-by-step instructions.