Now the leak has come from the display Symbian 3.0 Belle PR obtained from Forum Nokia.
As you might already know that the PR3 of Symbian OS will have a more modern UI with UI display changes significantly. From the screenshot you can see, there are some changes to the calendar view and change the message editor.
For a while it all looks very neat and far better than the last version of Symbian, although this probably is not Symbian Nokia Belle but at least now have put a nicer UI than the previous version.
Although it could be that Nokia will change some of the UI before actually releasing it officially hits the market later, so let us wait and see what changes occur.
Source: Forum Nokia