Today at the Office Developer Conference in Redmond, Bill Gates announced the immediate availability of a Community Technology Preview of Visual Studio Tools for Office 'v3'. This CTP release works against the Beta 1 Technical Refresh of Microsoft Office 2007. If you don't have access to the Beta 1 Technical Refresh of Microsoft Office 2007, don't despair--there will be another release of the CTP coming later this spring that will work against Office 2007 Beta 2 which will be more widely available.
If you want to read a tutorial that talks about the VSTO 'v3' CTP, follow this link. Kathleen McGrath has also blogged with a video link showing the CTP in action.
The CTP is available for download here and it installs on top of an existing Visual Studio 2005 installation. Here's some more info: