There is no out of box way of getting slidedecks to open in presentation mode in SharePoint 2007 (or 2003), even if they are saved as pps/x files. This is related to browser and file association issues that are far too time consuming for most end users to sort out.
A much simpler solution : Save the slideshow as a PDF and upload it.
Thanks to Neels du Toit from Absa Capital for bringing this to our attention. 🙂
It works fine in SharePoint 2010.