Here's another resource that I'm going to add to my RibbonX table of contents... it's the list of Office 2007 Control ID's. This is a package of 24 Excel spreadsheets that list all of the Ribbon control names, types and even the tabs and groups that they reside in (if any). There's one file each for Excel, Access, Word, Powerpoint and the RTM updates, and the rest are all for Outlook.
If you are trying to figure out what a control's idMso is for Ribbon customization, these files will be invaluable to you. Just open them up, and start working through them with the filters to narrow down what tab your control is on, what group, etc... Very handy.
You may need to pass the Windows Genuine Advantage validation to get it, and be aware that it is an exe file that extracts into the goods. (Go figure that. You'd think they could just give you a zip file, but whatever.)
The file can be downloaded from this page on Microsoft's site.