This Thursday afternoon I attended the rerun of the VSTO session (Microsoft Visual Studio Tools for Microsoft Office System - quite a mouthfull, I'll stick with VSTO) in the Smart Client track (my main track was VSTE - Visual Studio Team Edition). Wow! The 2003 version didn't impress me at all when I saw it back then. But 2005 really rocks. Catherine Heller (Technical Evangelist) did a nice demo in Excel. The way the document is design- and programmable, access to the action pane and rolling your own smart tags is almost too easy. Forget about VBA, serious Office development is now on the horizon.
VSTO 2005 Blog
VSTO 2005 Beta Page
What's New in Visual Studio 2005 Tools for Office