I wanted to edit my initial post, but alas it may be too long ago I posted it.

The rebuild with mingw worked fine, i.e. I can run the mingw version of Qt Creator just fine and the new widgets show up and everything works as expected.
The documentation suggested to build outside of the src directory, which I did.

How do I now replace the Qt Creator from the \Qt\2010.01 folder? The rebuild created the following folders with files in them:

bin
lib
share
src