The VSTO 'v3' CTP for Office 2007 Beta 2 is available now.  Start playing with Office 2007 development and the next version of VSTO today.

First you need to get Office 2007 Beta 2: 

Then go get the VSTO 'v3' CTP for Office Beta 2--this is an add-on that installs on top of an existing VS 2005 install:

Have fun!