From: http://www.pcworld.com/news/article/0,aid,116005,00.asp

Future OS will track all your contacts regardless of the type of communication

Chief among Longhorn's capabilities is a communications history from which you can manage everything, says Paul Erickson, development manager for Windows communications. From this central location, you'll be able to sort and filter messages by various themes such as name of sender or date of conversation, he says.

For example, every time you get an e-mail from a particular person, a copy of that message ends up in the history file under the person's name. The next time you prepare to communicate with that person, Longhorn will automatically call up your message history. Later, if you use other methods to communicate with the same person, the OS will tie the resulting messages together so you can see all of them.