Gabor has a great blog post on how researchers are changing the ways we should approach information overload in our mail clients. He cites Outlook as a specific example of the 'old' three-pane view and discuss three new areas that researchers are focused on: task-based mail organization, smart organization structures, and contact maps.
Task-based mail organization is an extension of what we've tried to accomplish in Outlook 2007, where your tasks, flagged mails, and any other flagged item (RSS post, SharePoint doc, etc.) show up together in the new To-Do Bar. This is not an entirely new concept but one that many information organization apps are moving towards, focusing management not on individual mail items but by the tasks which they are associated with. This makes it easier to monitor entire workflows and group them by the common actions needed to accomplish them. While this is similar to Outlook's unified task and category management, it is still a departure from the traditional way that all mail apps up until now have taught users how to 'see' incoming mail and how to triage it.
As search technologies become faster and more intelligent we will begin to think about the automatic sorting of mail based on pre-defined and computer-learned heuristics. This allows the application to automatically pivot all of your data (mail, appointments, tasks, etc.) around different properties. Gabor's post mentions the ReMail, a test application from IBM about four years ago that was built on the concept of 'thread arcs'. It's actually a cool concept – get rid of all the space that's wasted by a standard tree view and represent it by showing an image (in this case a set of swirly connected 'arcs'). I know I'm constantly going through my mails and deleting the hundreds of responses in a long thread and just saving the most recent; perhaps graphical representations like this can help me manage the thread more effectively and also bubble-up the important information sooner.