I'll close this thread that I opened with  the solution I found for my problem.  Hopefully it may benefit others. And thanks to those who replied with their thoughts.

As is often the case, the error message was misleading in that it implied (or I assumed) that the problem was with the connection string.  I was using a user id and a password in the string BUT my local instance of SQL Server was set for Windows Authentication mode only, not Mixed Mode as it should have been..

The way I found the problem came from a suggestion that I try to log in to SQL Server using the same uid and pwd credentials.  That attempt failed wirh a login for user error message, specifically SQL Error 1846 State 1.  But that message doesn't tell the whole story either..  To find out what it "really" failed, you have to look at the SQL Error Log -- it said State 58 - that only Windows Authentication was allowed.  Once I reset the Authentication to Mixed Mode AND (very importantly) Stopped and Started the SQL Server service,  I was able to login in and sure enough my DNN installation succeeded. 

So the Connection String was OK.  SQL Server's authentication mode was the culprit.  Maybe if I had use Integrated Security = True in the string instead of uid and pwd it would have worked right away.

So the suggestion for the other user who had a similar problem is -- take a look at your SQL Authentication mode and be sure it's set for Mixed Mode.

Al