When my developers first encountered problems loading VSTO projects I visited the VSTO pages on Microsoft's site to verify the requirements. Our standard desktop is Office 2003 Standard. Although the pages did not specify Office Professional, I upgraded the developer PCs to Professional, and it solved our developer problems. We did not anticipate that users with Standard edition would have problems.
My Outlook addin is still in development, though I tested limited functionality on a VPC using Office Standard. To the extent that we tested it, we have not seen problems using our VSTO addin with Office 2003 Standard.
Now I'm hearing that VSTO might only be supported with Office 2003 professional. The explanation I've heard is that only Professional ships with all the necessary libraries, and that even though I have not had problems with Standard, I may eventually hit a brick wall when I add unsupported functionality.
Can anyone clarify, preferably someone acting in an official Microsoft capacity?