Be sure to call Microsoft and go over this with them. From what I see the tightening of the security seems to be at the root of the issue.
Let me be clear here. I did not offer you any fix but an observation that this is happening elsewhere but we don't intend to fix this in our office. We applaud any move that tightens security and will change our procedures to go with it.
I am having a problem with opening files in .pps and .ppsx from Internet Explorer. Currently, we have a mixed environment with Office 2003 and Office 2007. The PowerPoint shows are saved as such and will open in SlideShow view for the people that still have Office 2003. However, for the people with Office 2007, the PowerPoint show will open in Normal or Edit mode vice SlideShow view.
This happens with both IE6 and IE7.
I tried the steps in http://www.pptfaq.com/FAQ00189.htm but to no avail. I changed my registry settings, looked at my File Options, but it still does not work correctly.
I added the Mime type for .PPSX into my IIS server as application/vnd.openxmlformats-officedocument.presentationml.slideshow but it still opens in edit mode vice SlideShow mode.
Please offer more suggestions.