What's different on the 2 machines? Are you deploying all required
dependencies of your addin (including the VSTO runtime and Framework, the
Office and Outlook PIA's, etc.)?
Take a look at the following troubleshooting information for managed code
addins:
http://blogs.msdn.com/vsod/archive/2008/04/22/Troubleshooting-com-add-in-load-fa
ilures.aspx

Signature
Ken Slovak
[MVP - Outlook]
http://www.slovaktech.com
Author: Professional Programming Outlook 2007.
Reminder Manager, Extended Reminders, Attachment Options.
http://www.slovaktech.com/products.htm
> Our VSTO 2007 Addin is being immediately auto-disabled on one machine but
> not another on installation.
>
> Anyone know why this would be?