Why would you want to do that you ask? Well for developers, probably the main reason would be to rewrite or wrap up some ugly lotus notes application (or all of lotus notes for that matter) and expose it in outlook. But seriously, there is a good chance your end user might spend more time in Outlook then any other application they have installed. And then of course, outlook has some context that might be helpful. Maybe, just maybe, when a user wants to talk to a customer, they open up their contact details in outlook first. Nothing worst than having to then alt-tab to another application and lookup the same contact in another system, perhaps to lookup their order history, etc. Or maybe you'd just like to rip through the information in outlook (contacts, appointments, emails) and integrate that data with some other application.
The outlook support for VSTO 2005 was a well kept secret until yesterday at TechEd when this was demonstrated at the keynote. How long are you going to have to wait before you can get your hands on this? How fast is your internet connection?