The problem did not result from an Office update. Before I started uninstalling the security-related updates, I installed Open Office Calc on my computer and replicated the problem, which indicates that the problem originated outside MS Office. After I uninstalled KB2778930, both Excel and Calc made the DDE connection.
I made a count of all the Jan 9 Windows 7 updates that would have to be individually uninstalled (followed by a computer reboot for each) to test for some other update that might have been responsible. Outside of the Windows-specific updates, there were 81 others listed. There's no way I am going to spend days trying to track down another potential source for this problem, especially since I have already found that uninstalling KB2778930 fixes it.
Microsoft pushed out an update that caused the problem, now let them fix it. I've already given them a leg up in the process by finding KB2778930.
Which leads me to this question: Does my communication with you about this mean that this will be forwarded to Microsoft for resolution? If not, can you tell me how to let them know about it?
Thanks.