From QTBUG-8314:

Qt's binary packages should use the code signing process specific to Vista/Windows 7. For a little extra overhead, this would give users confidence that the Qt libraries come from us.

The signatures are in the files or resource bundle. On systems where signing is not used, it is ignored.

See following link for more information:

http://msdn.microsoft.com/en-us/library/ms537361(VS.85).aspx