If you haven't checked it out yet, Chris Kunicki has created a VSTO Zone at his site that has a bunch of useful information about VSTO:

http://www.officezealot.com/vsto/

Also, Kathleen has posted some more VSTO video tutorials:

Demo 5--How to Resize Excel Controls
Demo 6 - How to Add Controls to the Actions Pane
Demo 7 – How to Set the Stack Order of Controls on the Actions Pane
Demo 8 - How to Change Controls on the Actions Pane