With the release of outlook 2010 them nasty NK2 files are now gone. Outlook now stores these inside the users mailbox as "suggested contacts"
Because this data is stored in the users mailbox, if a user moves to another computer, the suggested contacts will follow them. With NK2 files the only way to ensure that this data followed the user around was to implement roaming profiles.
From testing I have verified that Outlook 2010 extends the attributes of a users mailbox on the exchange server adding in these extra required attributes. With exchange the attributes of a mailbox are controlled by the email client, not the exchange server. This is why when you create a mailbox on the server, the mailbox is never actually created unless an email client connects to the mailbox using MAPI. You can view these additional attributes using a MAPI editing program such as MFCMAPI.
I have verified on both Exchange 2007 and 2010 this data stores perfectly in the exchange mailbox and follows the user around regardless which computer they log into. I have not tested Exchange 2000/2003. If anyone could validate this for me please leave a comment.