I posted a screencast on Channel 9 today that walks through using a spreadsheet you download from the Microsoft Download center to identify an Office 2007 icon and then use that same Icon in your own Office 2007 add-ins.  In the screencast I use 'Orcas' Beta 2 to create a Word 2007 Template file with a custom ribbon bar that uses an Office 2007 icon for a button.  Hopefully this quick howto will show you how to take advantage of the existing Office icons in your add-in, enjoy!