Last week at the Office Developer Conference in Redmond, Bill Gates announced the availability of the March Community Technology Preview of Visual Studio Tools for Office 'v3'. 

Please do note upfront, this CTP only works the Beta 1 Technical Refresh of Microsoft Office 2007.  If you don't have the Beta 1 Technical Refresh of Microsoft Office 2007, there's a note below with more info; bottom-line is you'll have another shot later this spring.

If you want to get a hands-on tutorial that covers the VSTO 'v3' CTP, you can find it here.  Kathleen McGrath's blog also has a video that shows the CTP in action.

The CTP is available here for download. It installs with an existing Visual Studio 2005 installation. 

And some blurbs from the the official announcement:

This is the March 2006 Community Technology Preview of Microsoft Visual Studio Tools for Office "v3", planned for the next version of Microsoft Visual Studio ("Orcas").

NOTE: This CTP requires you to have the Beta 1 Technical Refresh of Microsoft Office 2007. Only customers currently registered with the Microsoft Office 2007 Beta 1 program (including the Office Developer Conference 2006 attendees) are able to install this CTP. If you are not one of them, there will be another release of this CTP coming later this spring that will be available for installation by the general public.