For a great example of the potential of Office Business Applications, check out the Accruent demo that's now up on Channel 9.
It's a recording of Ben Riga's presentation at the Worldwide Partner Conference this summer, in which he covers a wide variety of OBA development and integration opportunities: an Office front end featuring ribbon customization and customized task panes, a MOSS 2007 back end, Excel Services, dashboards, XPS and rights management, WPF, and even Virtual Earth and Windows Live Local. Ben has some more info on his blog.
Ignoring the functionality for a minute, just think about what it would be like to work all day in applications that look this good. Hey, how soon are we going to be doing expense reports this way at work?