This may be the wrong place to make this comment, but:
Recent problem with upgrade from 3.0.5 to 3.1 was eventually solved by the last post of a member - I had to change the site and blog ID number - it was set on "1". I set it for "2" - solved the problem immediately.
So, thank you all for your patience and unbelievably considerate support.
Your willingness to help resolve problems that dumbo's like me create is the stuff of legends.
Why did the problem occur - because WordPress was made to work better!