I have a user who attempts to edit a VSD file in a SharePoint 2010 library and Visio 2010 opens the document read-only. The library has versioning enabled, but check-outs are not required.
If I open the file using Visio 2003, it opens normally in write mode.
I have verified that the site is in the Trusted Locations list.
Sometimes the user is able to check-out the file from within Visio after opening it, and then can save and check-in the file. However, if he checks the file out first in SharePoint, this does not work.
-Jim