Something that I happened upon this week after reading one-to-many post saying that VSTO Add-ins can't be installed for all users under Office 2007.  I knew it worked that way under Office 2003, and they couldn't have broken compatability that much in '07.  I was determined to find a workable solution to this problem because the sheer administrative overhead of needing each user to run the install or another patch mechanism was rediculous.

First, I had to identify the major difference between '03 and '07 when dealing with VSTO:  '07 supports it natively.  In '03 all VSTO Add-ins are loaded via a generic shim and are essentially regular COM add-ins.  In '07 the add-ins are loaded directly by the Office app, ignoring the shim.  To get around this issue, you have to cause '07 to load your add-in the same way that '03 would:  as a COM-shimmed add-in.  Invoking this behavior is a simple stroke of the delete key.

Setup your installer as normal, except having all registry entries defined under HKLM instead of HKCU.  Then navigate to HKLM/Software/Microsoft/Office/[APP]/Addins/[YourAddin].  Remove the "Manifest" value from this key.  This value being missing causes '07 to load your add-in via the ProgID [YourAddin], which eventually leads it to the VSTO AddinLoader and your real add-in via the Manifest defined under your CLSID/[GUID] entry.

There is a downside to this method:  If your add-in crashes, it will take out all other add-in using the AddinLoader via Office's disabled list.