While Office has had something to offer to developers for some time, the upcoming release really does move things up a level. If you haven't seen some of the server capabilities - like being to able to run Excel on a server and have your users access it via a browser with no need for Excel on the client, or browser-based InfoPath forms, or integration of Windows Workflow Foundation with Sharepoint - it's worth sitting up to take note. And of course, there's plenty of innovation at the client side too with the new "ribbon" toolbar and XML file formats for example.
A good way to get a brain dump on this stuff is our upcoming Office 2007 developer event. Held over two days in Hammersmith, London, with speakers from the US and the UK, it dives deep into the developer opportunities for Office.
This is filling rapidly so register here now: www.microsoft.com/uk/business/devcon.