When the next version of Outlook appears next year, it will include at least one feature that appears to build on Gmail's threaded conversations idea: combining related (but separate) email conversations into a single item.
Threaded conversations tend to divide email users: Gmail enthusiasts think they're the only way to go, others see them as an unwanted intrusion. Outlook 2007 actually includes a basic Conversation view, but for Outlook 2010, there's further changes afoot, as the official Outlook blog reveals:
Not only will Outlook 2010 provide the ability to view, sort, and categorize emails as conversations – using tools like Clean Up and Ignore, people can simply combine related messages to reduce inbox clutter or ignore irrelevant e-mail conversations altogether.
Outlook 2010 won't go into technical beta until the third quarter of 2009, but we're keen to see just how that feature pans out. Other promised enhancements include MailTips (to remind people of common mistakes like forgetting to include attachments) and a better Web experience (though I doubt that will include actually making Outlook Web Access work properly in Firefox).
Outlook and Exchange 2010 - The Next Wave [ Microsoft Office Outlook Team Blog]