UPDATE>
The problem recurred in a copy of my backup!  I did track down the cause,
and wanted to publish it here.
My project contains Crystal Reports XIR2 reports.  These were created with
Business Objects CR XI R2 Developer Edition, which is installed on my box.  
When I upgraded from VS2005 Pro to VS 2005 Team Dev, I selected the Crystal
Reports option as part of VS2005.  This overwrote any references to the
installed Business Objects version, which is a later version.
When I would open a report within the IDE, I would lose all references to
dll's outside of my solution, and get this System.IO.PathTooLongException.

To fix the problem,
I UNINSTALLED Business Objects CRXI R2 Dev Edition.  

I repaired my VS2005 installation.  At this point, version warning were
generated whenever I opened a Crystal report.  Also, the proper merge module
could not be found in my install project.

I RE-INSTALLED Business Objects CRXI R2 Dev Edition.

Life is back to normal again.  

Moral of the story:  Anyone upgrading from one version of VS2005 to another
who has  Business Objects CRXI R2 on their box should remove it before the
upgrade process.  Alternatively, not selecting Crystal Reports when
installing VS2005 might also prevent the overwrite from happening, but I did
not test this approach.