Most people probably don't know that Outlook saves frequently used email addresses, which is completely separate from Outlook Contacts. Those addresses are very useful to migrate along with your other Outlook files over to Outlook 2010.
There are 3rd party tools out there that help to do the migration. But, I'm always one to try to use native tools first, as long as they work well. I found this tool from Microsoft that seems to work well:
http://support.microsoft.com/kb/980542#ImportNK2Files_Outlook
In addition, to this being a useful tool, the article also points out that Outlook 2010 no longer uses a .NK2 file to store these frequently used addresses. I didn't figure out where Outlook 2010 now stores these addresses. Perhaps there's some tie-in in the future to Microsoft's Live Services, in which case I would expect it to be in some sort of web-enabled file (i.e. XML or similar).
One thing I haven't figure out is if the machine is in a domain/Exchange, then what happens to those names that are not complete email addresses because of Exchange resolution in Outlook 2007. Do those names (i.e. usernames) get imported and become duplicates? I just did this for someone. We'll find out soon enough.
